Say how to test stdlib packages in CONTRIBUTING.md (#45509)

Because stdlib packages are in the julia sysimg, simply running Pkg.test will
not work as the sysimg version will shadow the one being developed.
Mention that the developer can change the package UUID temporarily to enable
Pkg to test a stdlib

CONTRIBUTING.md

@@ -249,6 +249,24 @@ system image before running the corresponding test. This can be useful as a shor
 on the command line (since tests aren't always designed to be run outside the
 runtest harness).
 
+### Contributing to the standard library
+
+The standard library (stdlib) packages are baked into the Julia system image.
+When running the ordinary test workflow on the stdlib packages, the system image
+version overrides the version you are developing.
+To test stdlib packages, you can do the following steps:
+
+1. Edit the UUID field of the `Project.toml` in the stdlib package
+2. Change the current directory to the directory of the stdlib you are developing
+3. Start julia with `julia --project=.`
+4. You can now test the package by running `pkg> test` in Pkg mode.
+
+Because you changed the UUID, the package manager treats the stdlib package as
+different from the one in the system image, and the system image version will
+not override the package.
+
+Be sure to change the UUID value back before making the pull request.
+
 ### Contributing to patch releases
 
 The process of creating a patch release is roughly as follows: