Add a CONTRIBUTING.md file to the root directory in rust

[catamorphism]

It's still a bit hard to find https://github.com/mozilla/rust/wiki/Note-development-policy , so some pull requests are coming in targeted to master. Eric Holmes pointed out that if you create a CONTRIBUTING.md file in the root, Github will automatically show the contents in the header when someone opens a pull request. This could be a link to Note-development-policy or the contents of it, either way.

[catamorphism]

Added in c880d0a - but I'll leave this open until we have a chance to merge to master, since I'm assuming it won't show up automatically when someone opens a pull request unless the file is on master.

[steveklabnik]

I can confirm that I opened a PR against master today because I did not see that document. :(

[catamorphism]

@steveklabnik Thanks for the report. Once the build stops being broken, I'll push incoming to master and that should fix everything.

[brson]

Looks done.