doc: add code scan badge

Author: fry69

README.md

@@ -2,6 +2,7 @@
 
 [![JSR](https://jsr.io/badges/@fry69/files-to-prompt-ts)](https://jsr.io/@fry69/files-to-prompt-ts)
 [![JSR Score](https://jsr.io/badges/@fry69/files-to-prompt-ts/score)](https://jsr.io/@fry69/files-to-prompt-ts/score)
+[![CodeQL](https://github.com/fry69/files-to-prompt-ts/actions/workflows/github-code-scanning/codeql/badge.svg)](https://github.com/fry69/files-to-prompt-ts/actions/workflows/github-code-scanning/codeql)
 
 A command-line tool to concatenate files and directories in a structured way to a single prompt for use with large language models and other applications.