add google group subscribtion method using only emails

* users don't have to create an google account for using the google groups. They
  can simply email to e.g. "docker-user+subscribe@googlegroups.com" to
  subscribe.

* since this behavior is not mentioned on the google group website, i think its
  a good idea to explain this method here.

Signed-off-by: toogley <toogley@mailbox.org>

CONTRIBUTING.md

@@ -154,6 +154,8 @@ However, there might be a way to implement that feature *on top of* Docker.
       The <a href="https://groups.google.com/forum/#!forum/docker-dev" target="_blank">docker-dev</a>
       group is for contributors and other people contributing to the Docker
       project.
+      You can join them without an google account by sending an email to e.g. "docker-user+subscribe@googlegroups.com".
+      After receiving the join-request message, you can simply reply to that to confirm the subscribtion.
     </td>
   </tr>
   <tr>

README.md

@@ -234,6 +234,8 @@ We are always open to suggestions on process improvements, and are always lookin
       The <a href="https://groups.google.com/forum/#!forum/docker-dev" target="_blank">docker-dev</a>
       group is for contributors and other people contributing to the Docker
       project.
+      You can join them without an google account by sending an email to e.g. "docker-user+subscribe@googlegroups.com".
+      After receiving the join-request message, you can simply reply to that to confirm the subscribtion.
     </td>
   </tr>
   <tr>