Add note that contributors may request reimbursement through OpenCollective (#9933)

CONTRIBUTING.md

@@ -10,6 +10,8 @@ You can contribute in the following ways:
 - Contributing code to Mastodon by fixing bugs or implementing features
 - Improving the documentation
 
+If your contributions are accepted into Mastodon, you can request to be paid through [our OpenCollective](https://opencollective.com/mastodon).
+
 ## Bug reports
 
 Bug reports and feature suggestions can be submitted to [GitHub Issues](https://github.com/tootsuite/mastodon/issues). Please make sure that you are not submitting duplicates, and that a similar report or request has not already been resolved or rejected in the past using the search function. Please also use descriptive, concise titles.

README.md

@@ -80,7 +80,7 @@ A **Vagrant** configuration is included for development purposes.
 
 Mastodon is **free, open source software** licensed under **AGPLv3**.
 
-You can open issues for bugs you've found or features you think are missing. You can also submit pull requests to this repository, or submit translations using Weblate. To get started, take a look at [CONTRIBUTING.md](CONTRIBUTING.md)
+You can open issues for bugs you've found or features you think are missing. You can also submit pull requests to this repository, or submit translations using Weblate. To get started, take a look at [CONTRIBUTING.md](CONTRIBUTING.md). If your contributions are accepted into Mastodon, you can request to be paid through [our OpenCollective](https://opencollective.com/mastodon).
 
 **IRC channel**: #mastodon on irc.freenode.net