Merge #13165: doc: Mention good first issue list in CONTRIBUTING.md

a508091 doc: Mention good first issue list in CONTRIBUTING.md (fanquake)

Pull request description:

Tree-SHA512: a954cda62d6dc8ab78809565be392a1c9f55c543e8d5c1a5a8588c0062e1a11bd4968991646e4aa5bd9b9761b1765fde4ceab8fea7a973148122ff249d728b92

Author: MarcoFalke

CONTRIBUTING.md

@@ -14,6 +14,9 @@ purposes. As such there are repository "maintainers" who are responsible for
 merging pull requests as well as a "lead maintainer" who is responsible for the
 release cycle, overall merging, moderation and appointment of maintainers.
 
+If you're looking for somewhere to start contributing, check out the
+[good first issue](https://github.com/bitcoin/bitcoin/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22)
+list.
 
 Communication Channels
 ----------------------