Change and Release Tracking in README.md and CHANGELOG.md

As described in https://github.com/firebaseco/mongoose-attachments/issues/15

* added section on stable release to README with links to 0.0.4 and a note that the next release is 0.1.0.
* added CHANGELOG.md with lists of issues (with references to gitub issues) for the next release, for 0.0.4, 0.0.3 and 0.0.2.