Add development instructions to CONTRIBUTING.md

CONTRIBUTING.md

@@ -10,6 +10,24 @@ Contributions to this project are [released](https://help.github.com/articles/gi
 
 Please note that this project is released with a [Contributor Code of Conduct][code-of-conduct]. By participating in this project you agree to abide by its terms.
 
+## Development and Testing
+
+Before you start, you may want to ensure that you have a recent version of node installed. You can see which version of node is installed on the actions runners here: https://help.github.com/en/actions/reference/software-installed-on-github-hosted-runners 
+
+### Common tasks
+
+* Transpile the TypeScript to JavaScript: `npm run build`.  Note that the JavaScript files are committed to git.
+* Run tests: `npm run test`.  You’ll need to ensure that the JavaScript files are up-to-date first by running the command above.
+* Run the linter: `npm run lint`.
+
+### Running the action
+
+To see the effect of your changes and to test them, push your changes in a branch and then look at the [Actions output](https://github.com/github/codeql-action/actions) for that branch.  You can also exercise the code locally by running the automated tests.
+
+### Integration tests
+
+As well as the unit tests (see _Common tasks_ above), there are integration tests, defined in `.github/workflows/integration-testing.yml`.  These are run by a CI check.  Depending on the change you’re making, you may want to add a test to this file or extend an existing one.
+
 ## Submitting a pull request
 
 1. [Fork][fork] and clone the repository