diff --git a/pdfminer/cmap.py b/pdfminer/cmap.py
index f0ea641..695ad51 100644
--- a/pdfminer/cmap.py
+++ b/pdfminer/cmap.py
@@ -1,4 +1,16 @@
 #!/usr/bin/env python
+
+""" Adobe character mapping (CMap) support.
+
+CMaps provide the mapping between character codes and Unicode
+code-points to character ids (CIDs).
+
+More information is available on the Adobe website:
+
+  http://opensource.adobe.com/wiki/display/cmap/CMap+Resources
+
+"""
+
 import sys
 import re
 import os