Merge pull request #3 from fjsj/patch-1

Closes #2: add note about Heroku deployment on doc

doc/index.rst

@@ -177,6 +177,24 @@ correctly. Instead, you'll need to do a three-step migration dance:
    old field's name.
 
 
+Note about deploying to Heroku
+------------------------------
+
+An important caveat when deploying an app dependent of `Fernet` to Heroku: you
+need to specify all requirements (even dependencies of dependencies) explicitly.
+In general, this is a good practice for version pinning purposes. But it's
+necessary for Fernet on Heroku because it depends on `cryptography`_ library,
+which in turn depends on libffi, a C library. When cryptography is explicitly
+defined on requirements.txt, `Heroku knows`_ it depends on libffi and installs it.
+
+Therefore, an easy solution is to freeze your requirements after installing `Fernet`::
+
+    pip freeze > requirements.txt
+
+.. _cryptography: https://cryptography.io/en/latest/
+.. _Heroku knows: https://github.com/heroku/heroku-buildpack-python/blob/master/bin/steps/cryptography
+
+
 Contributing
 ------------