Added description of apidoc_template_dir option to README

Author: itsmoosh

README.rst

@@ -58,6 +58,15 @@ The *apidoc* extension uses the following configuration values:
 
    **Optional**, defaults to ``api``.
 
+``apidoc_template_dir``
+   A directory containing user-defined templates. Template files in this 
+   directory that match the default apidoc templates (``module.rst_t``, 
+   ``package.rst_t``, ``toc.rst_t``) will overwrite them. The default templates
+   can be found in ``site-packages/sphinx/templates/apidoc/``. This path is 
+   relative to the documentation source directory.
+
+   **Optional**, defaults to ``templates``.
+
 ``apidoc_excluded_paths``
    An optional list of modules to exclude. These should be paths relative to
    ``apidoc_module_dir``. fnmatch-style wildcarding is supported.