Add pygit2 instructions to README

README.md

@@ -9,3 +9,15 @@ that are synced via a gossip protocol.
 The current version uses libgit2 as a storage backend, but doesn't use many git
 primitives, beyond git's objects as a storage medium. This may change in the
 future.
+
+pygit2
+------
+
+groundstation uses pygit2 under the hood for it's git interactions, and some
+features that it depends on have not landed in a stable release yet.
+
+For this reason, you'll need to build and install the latest [libgit2](1) and
+[pygit2](2) from source.
+
+[1]:http://libgit2.github.com/
+[2]:https://github.com/libgit2/pygit2#building-on-nix-including-os-x