Auto merge of #973 - fitzgen:help-wanted, r=fitzgen

Add a link to issues labeled "help wanted" to CONTRIBUTING.md

Author: bors-servo

CONTRIBUTING.md

@@ -49,6 +49,7 @@ issue, provide us with:
 
 * [Issues labeled "easy"](https://github.com/rust-lang-nursery/rust-bindgen/issues?q=is%3Aopen+is%3Aissue+label%3AE-easy)
 * [Issues labeled "less easy"](https://github.com/rust-lang-nursery/rust-bindgen/issues?q=is%3Aopen+is%3Aissue+label%3AE-less-easy)
+* [Issues labeled "help wanted"](https://github.com/rust-lang-nursery/rust-bindgen/labels/help%20wanted)
 * Still can't find something to work on? [Drop a comment here](https://github.com/rust-lang-nursery/rust-bindgen/issues/747)
 
 ## Building