docs: add git config items to docs

Author: madmini

CONTRIBUTING.md

@@ -34,6 +34,12 @@ When making changes to the firmware:
 - Consider if your feature aligns with the [InfiniTime vision](doc/InfiniTimeVision.md)
 - Test your changes on a PineTime or InfiniSim
 - Make sure the code conforms to the [coding conventions](doc/coding-convention.md)
+- Configure Git to automatically fix whitespace issues:
+  
+  ```sh
+  git config set core.whitespace trailing-space,space-before-tab
+  git config set apply.whitespace fix
+  ```
 
 You can also check the [maintainer's guide](doc/maintainer-guide.md)
 for what maintainers will look at in PRs.