Update README.rst

the release notes in the github wiki seem to be no longer maintained.
Merge pull request #1869 from robbyt/patch-1

Update README.rst
Delete the changelog.

Since it hasn't been updated in 2 years.
Merge pull request #1870 from Celc/patch-1

Delete the old changelog in favor of the README link to ReadTheDocs changelogs. Fixes #1870.
Added support for CopyDBSnapshot
Added unit tests covering RDS DBSnapshots methods
Merge pull request #1872 from juanfont/develop

Add support for RDS CopyDBSnapshot. Fixes #1872, #1865.