add a docstring for allmydata.crypto.

src/allmydata/crypto/__init__.py

@@ -0,0 +1,8 @@
+"""
+Helper functions for cryptograhpy-related operations inside Tahoe
+
+For the most part, these functions use and return objects that are
+documented in the `cryptography` library -- however, code inside Tahoe
+should only use these functions and not rely on features of any
+objects that `cryptography` documents.
+"""