ENH: Add bids.ext namespace package for subpackages

bids/ext/__init__.py

@@ -0,0 +1,41 @@
+"""
+The PyBIDS extension namespace package
+
+``bids.ext`` is reserved as a namespace for extensions to install into.
+To write such an extension, the following things are needed:
+
+1) Create a new package with the following structure (assuming setuptools)::
+
+    package/
+      bids/
+        ext/
+          __init__.py
+          EXTENSION/
+            __init__.py
+            ...
+      setup.cfg
+      setup.py
+
+  The important things to note are that the ``bids/`` directory must be
+  empty apart from ``ext/`` and ``bids/ext/`` must be empty except for
+  your extension and an ``__init__.py``.
+
+2) Place the following (and nothing else) in ``__init__.py``::
+
+    __path__ = __import__('pkgutil').extend_path(__path__, __name__)
+
+3) Include the following lines in ``setup.cfg``::
+
+    [options]
+    install_requires =
+        pybids >= 0.15
+    packages = find_namespace:
+
+    [options.packages.find]
+    include =
+        bids.ext.EXTENSION
+        bids.ext.EXTENSION.*
+
+"""
+
+__path__ = __import__('pkgutil').extend_path(__path__, __name__)