add initial versioning policy

[mattsb42-aws]

As discussed in #129, I would like to propose that python-jose officially state what versioning policy the project is following. While there seems to be tentative agreement that the project is following SemVer already, it would be great if that could be explicitly stated. This will make it much simpler for consumers of this library to reason about how to handle upgrades and will help the maintainers in deciding how changes should manifest in new versions.

[codecov]

Codecov Report

Merging #131 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #131   +/-   ##
=======================================
  Coverage   96.52%   96.52%           
=======================================
  Files          14       14           
  Lines        1065     1065           
=======================================
  Hits         1028     1028           
  Misses         37       37

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[zejn]

Looks good.