Automate commit testing with proper stashing

Add a installation instructions for a git hook to automate testing
done on each commit.  Promotes proper handling of edge cases
related to testing code that will not be in the commit, which masks
errors that show up later as a nasty surprise.

Fully opt in, links to a gist on github.

README.md

@@ -28,6 +28,18 @@ $ cd git2-rs
 $ cargo build
 ```
 
+### Automating Testing
+
+Running tests and handling all of the associated edge cases on every commit
+proves tedious very quickly.  To automate tests and handle proper stashing and
+unstashing of unstaged changes and thus avoid nasty surprises, use the
+pre-commit hook found [here][pre-commit-hook] and place it into the
+`.git/hooks/` with the name `pre-commit`.  You may need to add execution
+permissions with `chmod +x`.
+
+
+To skip tests on a simple commit or doc-fixes, use `git commit --no-verify`.
+
 ## Building on OSX 10.10+
 
 Currently libssh2 requires linking against OpenSSL, and to compile libssh2 it
@@ -55,3 +67,5 @@ at your option.
 Unless you explicitly state otherwise, any contribution intentionally submitted
 for inclusion in git2-rs by you, as defined in the Apache-2.0 license, shall be
 dual licensed as above, without any additional terms or conditions.
+
+[pre-commit-hook]: https://gist.github.com/glfmn/0c5e9e2b41b48007ed3497d11e3dbbfa