Merge rust-bitcoin#2819: docs: Add more info to the update api files section

0ae5be3 docs: Add more info to the update api files section (Tobin C. Harding)

Pull request description:

  Done to try to further help new contributors and save maintainer time.

  Add an example of using `just check-api` and an example `git` command for devs to create the api patch now required for all changes to the public API.

ACKs for top commit:
  apoelstra:
    ACK 0ae5be3

Tree-SHA512: 042dad94c1887fcfad535b1eb51cb38a450f3928742d0887c8470888ba53cf79687b65cf7040bdcbe4ce4d49a843f66f198139c22d4f3c4d2689b94b946e6929

Author: apoelstra

CONTRIBUTING.md

@@ -200,6 +200,13 @@ All PRs that change the public API of `rust-bitcoin` must include a patch to
 the `api/` text files. This should be a separate, final patch to the PR
 that is the diff created by running `./contrib/check-for-api-changes.sh`.
 
+Or use `just`:
+
+```bash
+just check-api
+git commit -a -m 'api: Run just check-api'
+```
+
 ### Policy
 
 We have various `rust-bitcoin` specific coding styles and conventions that are