pythonGH-139040: Add Dev Container instructions in JIT README (python#139041)

Author: savannahostrowski

Tools/jit/README.md

@@ -51,6 +51,10 @@ Alternatively, you can use [chocolatey](https://chocolatey.org):
 choco install llvm --version=19.1.0
 ```
 
+### Dev Containers
+
+If you are working on CPython in a [Codespaces instance](https://devguide.python.org/getting-started/setup-building/#using-codespaces), there's no 
+need to install LLVM as the Fedora 41 base image includes LLVM 19 out of the box.
 
 ## Building