[clang-tools-extra] Update clang tidy Contributing guide (PR #106672)

[Aaron Ballman via cfe-commits]

================
@@ -646,10 +658,13 @@ directory.  The path to this directory is available in a lit test with the varia
 Out-of-tree check plugins
 -------------------------
 
+
 Developing an out-of-tree check as a plugin largely follows the steps
-outlined above. The plugin is a shared library whose code lives outside
+outlined above, including creating a new module and doing the hacks to 
+register the module. The plugin is a shared library whose code lives outside
 the clang-tidy build system. Build and link this shared library against
-LLVM as done for other kinds of Clang plugins.
+LLVM as done for other kinds of Clang plugins. If using CMake, use the keyword
+MODULE while invoking add_library or llvm_add_library.
----------------
AaronBallman wrote:

```suggestion
``MODULE`` while invoking ``add_library`` or ``llvm_add_library``.
```

https://github.com/llvm/llvm-project/pull/106672