Document minimum git version

[deviantintegral]

I was building on a server with Ubuntu 16.04 and it ships with git 1.9.1. In those cases, applying patches to non-git dependencies silently fails as no error message or exit code is returned by git apply.

This PR just updates the README; IMO it's probably too much complexity to parse git --version given the format of the string isn't stable.

[cweagans]

👍 good call. I think we're going to start moving in the direction of using patch for everything in the near future (just for the sake of predictable behavior), so hopefully this won't be a concern for too much longer.

[coveralls]

Coverage decreased (-13.8%) to 24.449% when pulling d3b342f on deviantintegral:document-git-version into d3b9741 on cweagans:master.

[stucki]

This should not be necessary anymore after merging #227.

[cweagans]

I've made a note to capture this and/or #227 in #447.