Move changelog to a separate file

[rth]

Hoping to help with #670, this moves the changelog from the README to a separate file (RELEASES.md similarly to rust repo as it is getting quite long.

Also,

• converts .rst to .md
• Improve section titles (include date) for newer releases
• Add some of the changes for 0.13 but clearly there is much more of those that can be addressed in a separate PR
• Add subsections for 0.13 to better see new feature, API changes etc.

[LukeMathWalker]

I think this was long overdue given the size of our README 😅
I'd like to keep the latest release changelog in the README though (0.13 - to be released) - what do you think @rth?

[rth]

I'd like to keep the latest release changelog in the README though (0.13 - to be released

Then maybe have it repeated both in the changelog and the readme? If there is a changelog, users are likely to look at it for the latest release as well?

[LukeMathWalker]

I'd like to keep the latest release changelog in the README though (0.13 - to be released

Then maybe have it repeated both in the changelog and the readme? If there is a changelog, users are likely to look at it for the latest release as well?

Duplicating it seems to be the best solution, I agree.

[LukeMathWalker]

Merging - we can then add the changelog for the latest release on the README before cutting 0.13.