Add line about using check_symbol.sh to CONTRIBUTING.md

CONTRIBUTING.md

@@ -16,6 +16,12 @@ Easy...
 
 1. Whenever adding a new part, use official [KiCad Library Utilities](https://github.com/kicad/kicad-library-utils) to check and fix libraries. There are some things that are not fully respected, such as using rounded rects for rectangular footprints. We use regular rects due to compatibility with some PCB milling tool path generating software. 
 
+1. At the moment we skip checking the PWR_FLAG symbol as KLC says it should have a power input pin, but in official KiCad libraries it has power output pin. Use the following to skip it.  
+   ```
+   ./check_symbol.py ../../fab.kicad_sym -p '^(?!.*PWR_FLAG).*$' -vs
+   ```
+   Notice the regex.
+
 1. Do a `git push` to the repository if you have access or use the merge request feature. If you do not know what a merge request is, read about [GitLab merge requests here](https://docs.gitlab.com/ee/user/project/merge_requests/creating_merge_requests.html).
 
 ## 3D Models