docs: add CONTRIBUTING.md (#3)

Author: kenjis

CONTRIBUTING.md

@@ -0,0 +1,10 @@
+# Contributing to CodeIgniter Coding Standard
+
+CodeIgniter Coding Standard is a community driven project and accepts contributions of
+code and documentation from the community.
+
+If you'd like to contribute, please read the [Contributing to CodeIgniter](https://github.com/codeigniter4/CodeIgniter4/blob/develop/contributing/README.md)
+guide in the [main repository](https://github.com/codeigniter4/CodeIgniter4).
+
+If you are going to contribute to this repository, please report bugs or send PRs
+to this repository instead of the main repository.