DOC: Add usage to ImageOpener docstring

Author: effigies

nibabel/openers.py

@@ -153,6 +153,18 @@ class ImageOpener(Opener):
 
     A trivial sub-class of opener to which image classes can add extra
     extensions with custom openers, such as compressed openers.
+
+    To add an extension, add a line to the class definition (not __init__):
+
+        ImageOpener.compress_ext_map[ext] = func_def
+
+    ``ext`` is a file extension beginning with '.' and should be included in
+    the image class's ``valid_exts`` tuple.
+
+    ``func_def`` is a `(function, (args,))` tuple, where `function accepts a
+    filename as the first parameter, and `args` defines the other arguments
+    that `function` accepts. These arguments must be any (unordered) subset of
+    `mode`, `compresslevel`, and `buffering`.
     """
     # Add new extensions to this dictionary
     compress_ext_map = Opener.compress_ext_map.copy()