Update CHANGELOG.md for v0.1.0-alpha

[arl]

❓ What

Once merged the commit will be tagged with v0.1.0-alpha.

🔨 How to test

1. List all steps necessary;
2. To test this pull request.

✅ Checklists

This section contains a list of checklists for common uses, please delete the checklists that are useless for your current use case (or add another checklist if your use case isn't covered yet).

• Is there unit/integration test coverage for all new and/or changed functionality added in this PR?
• Have the changes in this PR been functionally tested?
• Have new components been added to the related all.go files?
• Have new components been added to the documentation website?
• Has make gofmt-write been run on the code?
• Has make govet been run on the code? Has the code been fixed accordingly to the output?
• Have the changes been added to the CHANGELOG.md file?
• Have the steps in CONTRIBUTING.md been followed to update a Go module?

[codecov-io]

Codecov Report

Merging #172 (79a70af) into main (40a57bc) will increase coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #172      +/-   ##
==========================================
+ Coverage   53.95%   53.99%   +0.03%     
==========================================
  Files          60       60              
  Lines        4003     4006       +3     
==========================================
+ Hits         2160     2163       +3     
  Misses       1646     1646              
  Partials      197      197              

Impacted Files	Coverage Δ
filter/expand_list.go	94.87% <100.00%> (+0.42%)	⬆️

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[lucarin91]

LGTM 🎉

[tommyblue]

ok, the only thing I'd maybe do is to add a blank section for future changelogs, so that new PRs
will already have a clear way to add new changes:

## [Unreleased]

### Added

- 

### Changed

-

### Deprecated

-

### Removed

-

### Fixed

-

### Security

-

## [v0.1.0-alpha](https://github.com/AdRoll/baker/tree/v0.1.0-alpha) - 2021-03-19
...

[arl]

ok, the only thing I'd maybe do is to add a blank section for future changelogs, so that new PRs
will already have a clear way to add new changes:

I thought about that but I wanted to have a clean changelog for the release tag, and thought about adding the unreleased section after. But it's true that would mean having 2 PR per release.
Ok re-adding the empty unreleased section, and then merging