Add warning to TODO.

README.md

@@ -9,3 +9,9 @@ client from the browser!
 
 We will also have a hosted version of this, but it's not as secure as running
 your own. More info [here](https://www.etesync.com/faq/#web-client).
+
+# Important!
+
+There are some rough edges at the moment. For example, it uses a static IV
+instead of a secure random generated one! Please DO NOT use this for any real
+data and encryption keys until this warning is removed.