Make template override docs more clear

Closes #198

CHANGELOG.rst

@@ -14,6 +14,8 @@ Features
 
 Bug Fixes
 ^^^^^^^^^
+* `#198 <https://github.com/readthedocs/sphinx-autoapi/issues/198>`:
+  Documentation describes the required layout for template override directories.
 
 Trivial/Internal Changes
 ^^^^^^^^^^^^^^^^^^^^^^^^

docs/how_to.rst

@@ -20,6 +20,16 @@ It can be absolute, or relative to the root of the documentation directory
 
     autoapi_template_dir = '_autoapi_templates'
 
+Your template directory must to follow the same layout as the default templates.
+For example, to override the Python class and module templates:
+
+.. code-block:: none
+
+    _autoapi_templates
+    └── python
+        ├── class.rst
+        └── module.rst
+
 
 How to Customise the Index Page
 -------------------------------