Document mimetypes.MimeTypes.add_type() (python#122301)

Co-authored-by: Adam Turner <9087854+AA-Turner@users.noreply.github.com>

Doc/library/mimetypes.rst

@@ -295,3 +295,13 @@ than one MIME-type database; it provides an interface similar to the one of the
       types, else to the list of non-standard types.
 
       .. versionadded:: 3.2
+
+
+   .. method:: MimeTypes.add_type(type, ext, strict=True)
+
+      Add a mapping from the MIME type *type* to the extension *ext*. When the
+      extension is already known, the new type will replace the old one. When the type
+      is already known the extension will be added to the list of known extensions.
+
+      When *strict* is ``True`` (the default), the mapping will be added to the
+      official MIME types, otherwise to the non-standard ones.