Update contributing.md - contributors should test first before PR. (#370)

Update contributing.md

Author: cocoindex-dev

docs/docs/about/contributing.md

@@ -51,6 +51,7 @@ Following the steps below to get cocoindex build on latest codebase locally - if
     ```
 
 ## Submit Your Code
+CocoIndex is committed to the highest standards of code quality. Please ensure your code is thoroughly tested before submitting a PR.
 
 To submit your code: