Update reference to libgit2 version in README.

Author: ehuss

CONTRIBUTING.md

@@ -18,6 +18,7 @@ The following steps can be used to update libgit2:
     * Update the dependency version in [`Cargo.toml`](https://github.com/rust-lang/git2-rs/blob/master/Cargo.toml) to match the version in the last step (do not include the `+` metadata).
       Also update the version of the `git2` crate itself so it will pick up the change to `libgit2-sys` (also keeping in mind if it is a SemVer breaking release).
     * Update the version in [`README.md`](https://github.com/rust-lang/git2-rs/blob/master/README.md) if needed.
+      There are two places, the `Cargo.toml` example and the description of the libgit2 version it binds with.
     * If there was a SemVer-breaking version bump for either library, also update the `html_root_url` attribute in the `lib.rs` of each library.
 3. Run tests.
    `cargo test -p git2 -p git2-curl` is a good starting point.

README.md

@@ -16,9 +16,10 @@ stable release as well.
 
 ## Version of libgit2
 
-Currently this library requires libgit2 1.4. The source for libgit2 is
-included in the libgit2-sys crate so there's no need to pre-install the libgit2
-library, the libgit2-sys crate will figure that and/or build that for you.
+Currently this library requires libgit2 1.6.3 (or newer patch versions). The
+source for libgit2 is included in the libgit2-sys crate so there's no need to
+pre-install the libgit2 library, the libgit2-sys crate will figure that and/or
+build that for you.
 
 ## Building git2-rs