Readme: Mention the pkg-config dependency when building against the bundled libgit2.
Readme: Mention the pkg-config dependency when building against the bundled libgit2.

Fixes #402.

tpickett66/rugged,saraid/rugged,libgit2/rugged,tpickett66/rugged,libgit2/rugged,saraid/rugged,Acidburn0zzz/rugged,tpickett66/rugged,libgit2/rugged,Acidburn0zzz/rugged,saraid/rugged,Acidburn0zzz/rugged