Update CONTRIBUTING.md

add swift

CONTRIBUTING.md

@@ -23,6 +23,7 @@ If you have an idea for a query that you would like to share with other CodeQL u
       * Python: `python/ql/src`
       * Ruby: `ruby/ql/src`
       * Go: `go/ql/src`
+      * Swift: `swift/ql/src`
 
     Each language-specific directory contains further subdirectories that group queries based on their `@tags` or purpose.
     - Experimental queries and libraries are stored in the `experimental` subdirectory within each language-specific directory in the [CodeQL repository](https://github.com/github/codeql). For example, experimental Java queries and libraries are stored in `java/ql/src/experimental` and any corresponding tests in `java/ql/test/experimental`.