docs: there may be multiple BUILD files in a directory

docs/docs/using-pants/key-concepts/targets-and-build-files.mdx

@@ -60,6 +60,21 @@ python_distribution(
 )
 ```
 
+### Multiple BUILD files in a directory
+
+Typically, there would be one BUILD file in every directory containing source code and any other resources you
+may want to use as part of your builds. Most likely, having just one BUILD file in a directory is also what you would want.
+However, you can have multiple BUILD files in a single directory, if desired. When running a Pants goal, the contents
+of the BUILD files will be merged making it possible to better group your targets.
+
+Storing targets in multiple BUILD files also makes it possible to dynamically include or exclude targets from your
+builds. For example, you could include some experimental targets when running a Pants goal from a command line by
+extending the list of recognized BUILD file patterns:
+
+```bash
+$ pants --build-patterns="+['BUILD.experimental']" package project:app
+```
+
 ## Target addresses
 
 A target is identified by its unique address, in the form `path/to/dir:name`. The above example has the addresses `helloworld/greet:tests` and `helloworld/greet:bin`.