Add rebaselining steps to CONTRIBUTING

Author: MattKotsenas

CONTRIBUTING.md

@@ -5,3 +5,14 @@ We welcome contributions. If you want to contribute to existing issues, check th
 [good first issue](https://github.com/rjmurillo/moq.analyzers/labels/good%20first%20issue) items in the backlog.
 
 If you have new ideas or want to complain about bugs, feel free to [create a new issue](https://github.com/rjmurillo/moq.analyzers/issues/new).
+
+## Updating SquiggleCop baselines
+
+To update SquiggleCop baselines, run this command and review and commit the results:
+
+```powershell
+dotnet clean && dotnet build /p:PedanticMode=true /p:SquiggleCop_AutoBaseline=true
+```
+
+`$(PedanticMode)` turns on the CI configuration (e.g. `TreatWarningsAsErrors`) and `$(SquiggleCop_AutoBaseline)`
+automatically accepts the new baseline.