add how to claim an issue (gomods#669)

CONTRIBUTING.md

@@ -2,6 +2,9 @@ Hurray! We are glad that you want to contribute to our project! 👍
 
 If this is your first contribution, not to worry! We have a great [tutorial](https://www.youtube.com/watch?v=bgSDcTyysRc) to help you get started, and you can always ask us for help in the `#athens` channel in the [gopher slack](https://invite.slack.golangbridge.org/). We'll give you whatever guidance you need.
 
+## Claiming an issue
+If you see an issue that you'd like to work on, please just post a comment saying that you want to work on it. Something like "I want to work on this" is fine.
+
 ## Verify your work
 Run `make verify test-unit test-e2e` to run all the same validations that our CI process runs, such
 as checking that the standard go formatting is applied, linting, etc.