Added installing on OS X

This should be considered as a primer to get you started on using pygit2 on OS X. Since I only have one Mac, I could not verify this on more OS Versions and / or settings.
This would then require you to set the path every time you want to use something which uses pygit2. A more common solution should be to `brew link libgit2` which makes libgit2 available through `/usr/local/`, which is the path where non-system-provided software and libraries are installed and where the build system expects to see it (and where python will look when it needs to load libgit2).