Added a max limit of 8 on length parameter. Updated documentation.
diff --git a/docs/exceptions.rst b/docs/exceptions.rst
index 1e31e31..8ca9df2 100644
--- a/docs/exceptions.rst
+++ b/docs/exceptions.rst
@@ -36,3 +36,9 @@
 
     This is raised when the verify method of a key derivation function's
     computed key does not match the expected key.
+
+
+.. class:: InvalidToken
+
+    This is raised when the verify method of a one time password function's
+    computed token does not match the expected token.