Explain what "latest libgit2" means better

[carlosmn]

pygit2 targets the master branch, and "latest" isn't well-defined in
this case, as libgit2's default branch is development.

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This looks to be the cause of confusion in http://stackoverflow.com/questions/16570217/installing-pygit2-libgit2-in-python-virtualenv-on-a-mac and being explicit about what we target about is good anyway.

[heynemann]

Nice. This should help improve on the experience of installing pygit2.

I wonder whether it would also be a good idea to explain how to install it using brew.

What I did to start using pygit2 was:

$ brew install libgit2
$ pip install pygit2

And it just works. The version in homebrew is fairly recent (features even cloning repositories). Not sure of the impact it has in other features, though.