improve docs for Gentoo Linux installation

Changes summary:
    * Added available USE flags table(description included).
    * Added some tips regarding the use of docker in the Gentoo environment.
    * Sorted out docker installation ways.
    * Added information about the newly created Gentoo Docker team.
    * Misc improvements

Signed-off-by: Pavlos Ratis <dastergon@gentoo.org>

docs/sources/installation/gentoolinux.md

@@ -4,53 +4,77 @@ page_keywords: gentoo linux, virtualization, docker, documentation, installation
 
 # Gentoo
 
-Installing Docker on Gentoo Linux can be accomplished using one of two
-methods. The first and best way if you're looking for a stable
-experience is to use the official app-emulation/docker package directly
-in the portage tree.
-
-If you're looking for a `-bin` ebuild, a live ebuild, or bleeding edge
-ebuild changes/fixes, the second installation method is to use the
-overlay provided at
-[https://github.com/tianon/docker-overlay](https://github.com/tianon/docker-overlay)
+Installing Docker on Gentoo Linux can be accomplished using one of two ways: the **official** way and the `docker-overlay` way.
+
+Official project page of [Gentoo Docker](https://wiki.gentoo.org/wiki/Project:Docker) team.
+
+## Official way
+The first and recommended way if you are looking for a stable  
+experience is to use the official `app-emulation/docker` package directly  
+from the tree.
+
+If any issues arise from this ebuild including, missing kernel 
+configuration flags or dependencies, open a bug 
+on the Gentoo [Bugzilla](https://bugs.gentoo.org) assigned to `docker AT gentoo DOT org` 
+or join and ask in the official
+[IRC](http://webchat.freenode.net?channels=%23gentoo-containers&uio=d4) channel on the Freenode network.
+
+## docker-overlay way
+
+If you're looking for a `-bin` ebuild, a live ebuild, or a bleeding edge
+ebuild, use the provided overlay, [docker-overlay](https://github.com/tianon/docker-overlay)
 which can be added using `app-portage/layman`. The most accurate and
 up-to-date documentation for properly installing and using the overlay
-can be found in [the overlay
-README](https://github.com/tianon/docker-overlay/blob/master/README.md#using-this-overlay).
+can be found in the [overlay](https://github.com/tianon/docker-overlay/blob/master/README.md#using-this-overlay).
 
-Note that sometimes there is a disparity between the latest version and
-what's in the overlay, and between the latest version in the overlay and
-what's in the portage tree. Please be patient, and the latest version
-should propagate shortly.
+If any issues arise from this ebuild or the resulting binary, including
+and especially missing kernel configuration flags or dependencies, 
+open an [issue](https://github.com/tianon/docker-overlay/issues) on 
+the `docker-overlay` repository or ping `tianon` directly in the `#docker` 
+IRC channel on the Freenode network.
 
 ## Installation
 
+### Available USE flags
+
+| USE Flag      | Default | Description |
+| ------------- |:-------:|:------------|
+| aufs          |         |Enables dependencies for the "aufs" graph driver, including necessary kernel flags.|
+| btrfs         |         |Enables dependencies for the "btrfs" graph driver, including necessary kernel flags.|
+| contrib       |  Yes    |Install additional contributed scripts and components.|
+| device-mapper |  Yes    |Enables dependencies for the "devicemapper" graph driver, including necessary kernel flags.|
+| doc           |         |Add extra documentation (API, Javadoc, etc). It is recommended to enable per package instead of globally.|
+| lxc           |         |Enables dependencies for the "lxc" execution driver.|
+| vim-syntax    |         |Pulls in related vim syntax scripts.|
+| zsh-completion|         |Enable zsh completion support.|
+
+USE flags are described in detail on [tianon's
+blog](https://tianon.github.io/post/2014/05/17/docker-on-gentoo.html).
+
 The package should properly pull in all the necessary dependencies and
-prompt for all necessary kernel options. The ebuilds for 0.7+ include
-use flags to pull in the proper dependencies of the major storage
-drivers, with the "device-mapper" use flag being enabled by default,
-since that is the simplest installation path.
+prompt for all necessary kernel options.
 
     $ sudo emerge -av app-emulation/docker
 
-If any issues arise from this ebuild or the resulting binary, including
-and especially missing kernel configuration flags and/or dependencies,
-[open an issue on the docker-overlay repository](
-https://github.com/tianon/docker-overlay/issues) or ping
-tianon directly in the #docker IRC channel on the freenode network.
-
-Other use flags are described in detail on [tianon's
-blog](https://tianon.github.io/post/2014/05/17/docker-on-gentoo.html).
+>Note: Sometimes there is a disparity between the latest versions 
+>in the official **Gentoo tree** and the **docker-overlay**.  
+>Please be patient, and the latest version should propagate shortly.
 
 ## Starting Docker
 
 Ensure that you are running a kernel that includes all the necessary
-modules and/or configuration for LXC (and optionally for device-mapper
-and/or AUFS, depending on the storage driver you`ve decided to use).
+modules and configuration (and optionally for device-mapper
+and AUFS or Btrfs, depending on the storage driver you've decided to use).
+
+To use Docker, the `docker` daemon must be running as **root**.  
+To use Docker as a **non-root** user, add yourself to the **docker** 
+group by running the following command:
 
+    $ sudo usermod -a -G docker user
+ 
 ### OpenRC
 
-To start the docker daemon:
+To start the `docker` daemon:
 
     $ sudo /etc/init.d/docker start
 
@@ -60,10 +84,10 @@ To start on system boot:
 
 ### systemd
 
-To start the docker daemon:
+To start the `docker` daemon:
 
-    $ sudo systemctl start docker.service
+    $ sudo systemctl start docker
 
 To start on system boot:
 
-    $ sudo systemctl enable docker.service
+    $ sudo systemctl enable docker