Update CONTRIBUTING.md (#535)

* Update CONTRIBUTING.md

Add a couple additional commands to contributing guide.

* Update CONTRIBUTING.md

Co-authored-by: Timothee Guerin <tiguerin@microsoft.com>
Co-authored-by: Mark Cowlishaw <markcowl@microsoft.com>
Co-authored-by: Timothee Guerin <timothee.guerin@outlook.com>

CONTRIBUTING.md

@@ -116,6 +116,26 @@ Rush change will ask for the following questions for each modified packages:
   - `patch`: A bug fix.
   - `none`: Not relevant to the consumer of the packages. For example some added tests.
 
+## Linting
+
+```bash
+rush lint
+```
+
+PR validation enforces linting rules for the repo. This
+command will run the linter on all packages.
+
+## Regenerate Samples
+
+```bash
+rush regen-samples
+```
+
+PR validation runs OpenAPI emitters on samples and compares them to known,
+reviewed, checked-in versions. If your PR would change the generated output,
+run this command to regenerate any samples and check those files in with
+your PR. Carefully review whether the changes are intentional.
+
 # Using VS Code
 
 ## Recommended extensions