Update changelog

[btromanova]

Update changelog before the 0.5.0 release.

[dansimau]

This is more of a code addition/enhancement, I think it should probably go in "Added".

We can also make it say:

* Propagate SIGINT/SIGTERM to Terraform on shut down (avoids stale locks) (#49)

[btromanova]

Moved, thanks.

[dansimau]

This reads more easily as a list:

**Releasing a new version**

1. Make sure the CHANGELOG is updated
2. Create and push a new tag named `x.x.x` (following [semver](https://semver.org/))
3. The [release action](https://github.com/uber/astro/actions?workflow=release) will automatically build and upload binaries

Also we can add:

**Making and testing changes to the release process**

1. Make your changes to the [release GitHub action workflow file](https://github.com/uber/astro/actions?workflow=release) and push it to a branch
2. Create a new tag pointing to the tip of the branch (name it: `0.0.0-dev`)
3. Push the tag to trigger the action. The action will use the updated config.
4. Delete the tag after testing (`git tag -d 0.0.0-dev; git push --delete origin 0.0.0-dev`)

Disclaimer: I haven't tested the instructions for "Testing the release process" so we should verify that it works before landing (e.g. that the tag name 0.0.0-dev works).

[dansimau]

Also I'm starting to think these runbooks should be in the wiki and we should link to there from the README instead of making the README really long.

[dansimau]

Beautiful changelog 💯