DOC: Clarify RSA encoding and decoding depend on the cryptography package (jpadilla#664)

* DOC: Clarify RSA encoding and decoding depend on the cryptography package

Followup to jpadilla#208

* DOC: Add a reference to the cryptography section in the installation file

Fixes missing reference in the previously edited file

Author: TPXP

docs/installation.rst

@@ -7,6 +7,9 @@ You can install ``PyJWT`` with ``pip``:
 
     $ pip install pyjwt
 
+
+.. _installation_cryptography
+
 Cryptographic Dependencies (Optional)
 -------------------------------------
 

docs/usage.rst

@@ -17,6 +17,8 @@ Encoding & Decoding Tokens with HS256
 Encoding & Decoding Tokens with RS256 (RSA)
 -------------------------------------------
 
+RSA encoding and decoding require the ``cryptography`` module. See :ref:`installation_cryptography`.
+
 .. code-block:: pycon
 
     >>> import jwt