Add BSD installation instructions.

Fixes tj#498.

Installation.md

@@ -41,6 +41,10 @@ Last, to use `git line-summary`, `git summary` and `git ignore-io`, you need to
 `column.exe` from a [msys2][1] installation from `folder-your-msys2-installed/usr/bin`
 to `folder-your-git-installed/usr/bin`.
 
+### BSD
+
+Use the instructions to build from source below. Make sure you are using `gmake` (GNU `make`) instead of `make`.
+
 ## Building from source
 
 Obtain the git-extras source by cloning [its GitHub repo](https://github.com/tj/git-extras.git) or downloading a tarball of a [release](https://github.com/tj/git-extras/releases). Then install it by doing `make install` from the source tree.