Expand intro paragraph in README

Author: s0

README.md

@@ -4,7 +4,30 @@ This repository provides Docker images and GitHub Actions
 for use in CodeQL courses
 on [Learning Lab](https://lab.github.com/).
 
+These actions allow you to specify workflows
+that can check that course participants' queries are correct,
+by running their queries against a well-known CodeQL database,
+and checking the results are as expected.
+Whatever the outcome,
+the action will post a comment on the commit which was pushed
+to add the queries.
 
+When a user's results are incorrect,
+the comment will include details of which results are missing,
+and which are superfluous,
+including links to the lines of source code on GitHub when possible.
+
+## Usage
+
+TODO
+
+## Example Courses
+
+There are currently no published courses that use this repository,
+when this changes, we'll add a list here.
+
+Feel free to add your own courses to this list!
+See [CONTRIBUTING.md](CONTRIBUTING.md).
 
 ## License