add initial changelog with info for 3.1.0

CHANGELOG.rst

@@ -0,0 +1,35 @@
+---------
+Changelog
+---------
+
+3.1.0 -- 2019-??-??
+^^^^^^^^^^^^^^^^^^^
+
+Major
+"""""
+
+* Isolate and flesh out cryptographic backends to enable independent operation.
+  `#114 <https://github.com/mpdavis/python-jose/issues/114>`_
+  `#129 <https://github.com/mpdavis/python-jose/pull/129>`_
+* Remove pyca/cryptography backend's dependency on python-ecdsa.
+  `#117 <https://github.com/mpdavis/python-jose/pull/117>`_
+* Remove pycrypto/dome backends' dependency on python-rsa.
+  `#121 <https://github.com/mpdavis/python-jose/pull/121>`_
+* Make pyca/cryptography backend the preferred backend if multiple backends are present.
+  `#122 <https://github.com/mpdavis/python-jose/pull/122>`_
+
+Bugfixes
+""""""""
+
+* Fix invalid RSA private key PKCS8 encoding by python-rsa backend.
+  `#120 <https://github.com/mpdavis/python-jose/pull/120>`_
+
+Housekeeping
+""""""""""""
+
+* Test each cryptographic backend independently in CI.
+  `#114 <https://github.com/mpdavis/python-jose/issues/114>`_
+  `#129 <https://github.com/mpdavis/python-jose/pull/129>`_
+  `#135 <https://github.com/mpdavis/python-jose/pull/135>`_
+* Add flake8 checks in CI.
+* Add CPython 3.7 and PyPy 3.5 testing in CI.