Update README.md

Turned the warning at the top into a warning box.

Made the section headers consistent.

I was going to add a section, but I realized it belongs somewhere else.  :)

Author: geoffw0

swift/README.md

@@ -1,12 +1,11 @@
 # Swift on CodeQL
 
-## Warning
+> [!WARNING]
+> The Swift CodeQL package is an experimental and unsupported work in progress.
 
-The Swift CodeQL package is an experimental and unsupported work in progress.
-
-##
+## Development
 
-## Building the Swift extractor
+### Building the Swift extractor
 
 First ensure you have Bazel installed, for example with
 
@@ -30,8 +29,6 @@ set up the search path
 in [the per-user CodeQL configuration file](https://docs.github.com/en/code-security/codeql-cli/using-the-codeql-cli/specifying-command-options-in-a-codeql-configuration-file#using-a-codeql-configuration-file)
 .
 
-## Development
-
 ### Code generation
 
 Run