Add About section to README

Author: swinton

README.md

@@ -1,6 +1,13 @@
 # `code-scanning-playground`
 > :wave: :earth_americas: a playground for **Code Scanning** :roller_coaster:
 
+## About
+This repo demonstrates GitHub's [Code Scanning feature](), using ESLint to generate a "code scanning alert" in the Security tab.
+
+![example](images/example.png)
+
+Register for the code scanning beta [here](https://github.com/features/security/advanced-security/signup).
+
 ## Resources
 * microsoft/sarif-tutorials: User-friendly documentation for the SARIF file format., https://github.com/microsoft/sarif-tutorials
 * SARIF support for code scanning - GitHub Docs, https://docs.github.com/en/github/finding-security-vulnerabilities-and-errors-in-your-code/sarif-support-for-code-scanning