Add documentation on Markdown linting to CONTRIBUTING.md

Signed-off-by: Douglas Stebila <dstebila@uwaterloo.ca>

skip-checks: true

Author: dstebila

CONTRIBUTING.md

@@ -90,6 +90,12 @@ or running the CI locally (as above):
 act workflow_call -W '.github/workflows/basic.yml'
 ```
 
+Markdown documentation is checked with [markdownlint-cli2](https://github.com/DavidAnson/markdownlint-cli2) during the [basic.yml](.github/workflows/basic.yml) job, using the rules in [.markdownlint.yml](.markdownlint.yml). Changes to Markdown files can be validated locally with the same command CI runs:
+
+```bash
+npx --yes markdownlint-cli2@0.22.1 --config .markdownlint.yml "**/*.md" "#build" "#**/build"
+```
+
 ### New features
 
 Any PR introducing a new feature is expected to contain a test of this feature