Fixes "Mention the difference between stable docs and latest docs." (#…

…2549)

(try to) Solve #1996 

I am not sure whether this is the best way, so please advise if you have a different idea (see comment in the original issue).

README.rst

@@ -20,12 +20,19 @@ circuits and running them against quantum computers and simulators.
 Installation and Documentation
 ------------------------------
 
-Documentation is available at `cirq.readthedocs.io <https://cirq.readthedocs.io>`_:
+Cirq documentation is available at `cirq.readthedocs.io <https://cirq.readthedocs.io>`_.
+
+Documentation for the latest **unstable** version of cirq (tracks the repository's master branch; what you get if you ``pip install cirq-unstable``), is available at `cirq.readthedocs.io/latest <https://cirq.readthedocs.io/en/latest/>`_.
+
+Documentation for the latest **stable** version of cirq (what you get if you ``pip install cirq``) is available at `cirq.readthedocs.io/stable <https://cirq.readthedocs.io/en/stable/>`_.
+
 
 - `Installation <https://cirq.readthedocs.io/en/stable/install.html>`_
 - `Documentation <https://cirq.readthedocs.io>`_
 - `Tutorial <https://cirq.readthedocs.io/en/stable/tutorial.html>`_
 
+
+
 Hello Qubit
 -----------