Auto merge of #482 - rectang:contribution-guidelines, r=jdm

Add basic contfibution guidelines to README

In #453, @pmj proposed bare-bones contribution guidelines, which were received favorably by @jrmuizel. This pull request insert's @pmj's proposed text into the top-level README.

README.md

@@ -9,3 +9,7 @@ Targets macOS 10.7 by default.
 To enable features added in macOS 10.8, set Cargo feature `mac_os_10_8_features`. To have both 10.8 features and 10.7 compatibility, also set `mac_os_10_7_support`. Setting both requires weak linkage, which is a nightly-only feature as of Rust 1.19.
 
 For more experimental but more complete, generated bindings take a look at https://github.com/michaelwu/RustKit.
+
+## Contributing
+
+If you wish to start contributing or even make a one-off change, simply submit a pull request with the code or documentation change and we'll go from there.