Documentation: Add Windows section in build instructions

It seems that new people go to the build instructions from the main
README, don't see Windows and are then stuck.

We do have instructions for Windows, but they aren't noted in the build
instructions, so new people get stuck thinking there's no way to build
on Windows.

Documentation/BuildInstructions.md

@@ -69,6 +69,11 @@ pkg_add bash gmp gcc git flock gmake sudo
 $ pkg add coreutils gmake bash sudo git
 ```
 
+#### Windows
+For Windows, you will require Windows Subsystem for Linux 2 (WSL2). [Follow the WSL2 instructions here.](https://github.com/SerenityOS/serenity/blob/master/Documentation/NotesOnWSL.md)
+Do note the ```Hardware acceleration``` and ```Note on filesystems``` sections, otherwise performance will be terrible.
+Once you have installed a distro for WSL2, follow the Linux prerequisites above for the distro you installed, then continue as normal.
+
 ### Build
 > Before starting, make sure that you have configured your global identity for git, or the first script will fail after running for a bit.