Update CHANGELOG.md for 0.7.0 release

[w-flo]

I didn't expect my PR to be merged and released so quickly, so thanks @Kogia-sima and @vthg2themax!

I prepared a changelog update for 0.7. I believe there were some formatting issues so I fixed them, let me know if that was actually on purpose.

Also, I think the "v0.7.0" tag is missing from github, so before merging this PR, the tag should be added to make the new link work. Edit: I spoke too soon, the tag has just appeared. :-)

0.5.0, 0.6.0 and 0.6.1 are missing from the changelog.

Commit message:

This might help some people who want to know how difficult a 0.6 -> 0.7 upgrade would be for them.

Also fix some markdown issues.

[vthg2themax]

Thank you for your helpful contributions!