docs: there may be multiple BUILD files in a directory

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

@@ -60,6 +60,31 @@ 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. However, you can have multiple BUILD files in a single directory.
+When running a Pants goal, the contents of the BUILD files will be merged making it possible to better group your
+targets, if desired. For instance, you could store source code targets and
+[macros](../../writing-plugins/macros.mdx) in different files to have better separation of concerns:
+
+```python tab={"label":"BUILD"}
+python_sources()
+python_tests(name="tests")
+```
+
+```python tab={"label":"BUILD.macros"}
+jinja_templates(sources=["*.j2"])
+```
+
+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`.