Add Changelog.md and Danger Github action to code review process

[pengyin-shan]

Fixes #3257

1. Add Danger gem and Github action to the roadmap

Note that Danger is for reference. We can discuss changing the Danger configuration. Currently, the suggestion/reminder is:

1. PR prefers <= 1000 lines
2. If PR change >= 50, suggest adding a test
3. Remind to update CHANGELOG
4. [WIP] in the title will make this PR to be a draft pr
5. #trivial means no changelog is needed

An example:

2. CHANGELOG.md is ready for use

The workflow would be to add the description and link to the issue (if any) to the CHANGELOG every time a PR is submitted. I created the 'Changed', 'Added' and 'Fixed' sections for categorization purposes, but we are feeling to change to other formats.
The person who's gonna do the release will add the release version as the header to the CHANGELOG. Later, developers will pull the CHANGELOG and continue to build above it.

Example:

By doing this, we will have a good record of the process, and whoever doing the release can use CHANGELOG as part of the release note.

We can also consider adding this step to the contribution guide.

[benjaminfaure]

Looking good, thank you.

Because it includes the CHANGELOG file, should we merge this one as soon as possible ?

[briri]

looks good Wendy. I'm just wondering if the test check should use the RSpec directory instead of /test/

[briri]

and agree with @benjaminfaure that once this is ready we should merge it so we can start using the Changelog immediately

[pengyin-shan]

Thank you @benjaminfaure @briri ! Will merge after all tests passed