Release 3.1.1 (plotly#1096)

* Updated CHANGELOG and README
* Added release process to contributing.md

Merge pull request plotly#674 from plotly/update-changelog

Update changelog.

Merge pull request plotly#658 from plotly/2.0.0

2.0.0

Merge pull request plotly#656 from plotly/frames-support

Improve `frames` support in graph_objs.py.

Merge pull request plotly#201 from plotly/update-image-endpoint

Move from genimage —> v2/images/

Merge pull request plotly#196 from plotly/delete_fullData

dont return fullData on widget events