Add label **good first issue** for issues

Author: winlinvip

CONTRIBUTING.md

@@ -1,5 +1,5 @@
 Welome to contribute to SRS!
 
 1. Please read **Contribution([CN](https://github.com/ossrs/srs/issues/2866#contribution) EN)** before file new PR.
-2. Please start from fixing some [issues](https://github.com/ossrs/srs/issues), then follow the [guide](https://github.com/ossrs/srs/wiki/HowToFilePR).
+2. Please start from fixing some [Issues: good first issue](https://github.com/ossrs/srs/issues?q=is%3Aopen+is%3Aissue+label%3A%22good+first+issue%22), then follow the [guide](https://github.com/ossrs/srs/wiki/HowToFilePR).
 3. We will review your PR ASAP.