Document (and prefer) pyjwt[crypto] req format (jpadilla#426)

Author: gthb

docs/installation.rst

@@ -12,11 +12,16 @@ Cryptographic Dependencies (Optional)
 
 If you are planning on encoding or decoding tokens using certain digital
 signature algorithms (like RSA or ECDSA), you will need to install the
-cryptography_ library.
+cryptography_ library. This can be installed explicitly, or as a required
+extra in the ``pyjwt`` requirement:
 
 .. code-block:: console
 
-    $ pip install cryptography
+    $ pip install pyjwt[crypto]
+
+The ``pyjwt[crypto]`` format is recommended in requirements files in
+projects using ``PyJWT``, as a separate ``cryptography`` requirement line
+may later be mistaken for an unused requirement and removed.
 
 .. _legacy-deps: