Add a CHANGELOG.md file to the project root

[cgillum]

Having a single file that tracks changes across versions will be helpful for users to understand what fixes were released across different builds. This will be very useful when doing servicing since going through the GitHub releases can be tedious.

• [ ] Add the initial file and include links to PRs/fixes since the previous release
• [ ] Create a PR template in this repo that includes a checkbox asking if the PR author has updated CHANGELOG.md.

See here for an example: https://github.com/microsoft/durabletask-mssql/blob/main/CHANGELOG.md.

Basically, it should look something like this. Note that I like to explicitly highlight external users that contributed PRs, when possible.

## v1.0.1

### Updates

* Fixed bug ([#198](https://github.com/microsoft/durabletask-java/issues/198))
* Fixed bug ([#199](https://github.com/microsoft/durabletask-java/issues/199))

## v1.0.0

### New

* New feature A ([#196](https://github.com/microsoft/durabletask-java/pull/196)) - contributed by [@user](https://github.com/user)
* New feature B ([#197](https://github.com/microsoft/durabletask-java/pull/197)) - contributed by [@user](https://github.com/user)

### Updates

* Fixed bug ([#187](https://github.com/microsoft/durabletask-java/issues/187))
* Updated dependency ([#188](https://github.com/microsoft/durabletask-java/issues/188))

### Breaking changes

* Renamed method from `Foo` to `Bar`