adds zenodo config (#3392)

Adds zenodo config.

.zenodo.json

@@ -0,0 +1,13 @@
+{
+    "description": "Cirq is a Python library for writing, manipulating, and optimizing quantum circuits and running them against quantum computers and simulators.",
+    "license": "Apache-2.0",
+    "title": "Cirq",
+    "upload_type": "software",
+    "creators": [
+        {
+            "affiliation": "Google Inc",
+            "name": "Quantum AI team and collaborators"
+        }
+    ],
+    "access_right": "open"
+}

README.rst

@@ -80,6 +80,23 @@ For questions about how to use Cirq post to
 `Quantum Computing Stack Exchange <https://quantumcomputing.stackexchange.com/>`__ with the
 `cirq <https://quantumcomputing.stackexchange.com/questions/tagged/cirq>`__ tag.
 
+How to cite Cirq
+----------------
+
+Cirq is uploaded to Zenodo automatically. Click on this badge [![DOI](https://zenodo.org/badge/DOI/10.5281/zenodo.4062499.svg)](https://doi.org/10.5281/zenodo.4062499) to see all the citation formats for all versions.
+
+An equivalent BibTex format reference is below for all the versions:
+
+@software{quantum_ai_team_and_collaborators_2020_4062499,
+  author       = {Quantum AI team and collaborators},
+  title        = {Cirq},
+  month        = Oct,
+  year         = 2020,
+  publisher    = {Zenodo},
+  doi          = {10.5281/zenodo.4062499},
+  url          = {https://doi.org/10.5281/zenodo.4062499}
+}
+```
 
 Cirq Contributors Community
 ---------------------------