Add release date to changelog

[humitos]

It would be very helpful to have the date of the release in the changelog published at http://docs.readthedocs.io/en/latest/changelog.html

Code ref: https://github.com/rtfd/readthedocs.org/blob/8fe25841803e31a559c1b3ddb12d7f06cdeb0674/tasks.py#L62

[irkartik]

shall I take up this issue? @humitos

[RichardLitt]

@rajujha373 If you have a suggestion for how the code should look, open a PR! :D

[stsewd]

This was implemented on #3788 (merged)