Improvements to the readme.md

[jeherve]

Right now, the repo's readme.md is a copy of the plugin's readme.txt, and we rely on the wp-readme-to-markdown grunt plugin to keep the 2 files in sync.

While that makes sense, I'd like to offer some improvements:

### Tasks
- [ ] Extract changelog
- [ ] Add Security section
- [ ] Add Join us section
- [ ] Add Contributing / How to build section

See Details below.

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• The changelog is getting long, and I don't know if it is very useful for folks landing in this repo to see the full changelog. Maybe we could link to a changelog.md file instead, or to the readme.txt's changelog section?
• I think it would be nice to add 3 new sections to the readme.md file:

1. a "Security" section that would help folks know what to do if they want to report a security issue. Something like this:

## Security

Need to report a security vulnerability? Go to [https://automattic.com/security/](https://automattic.com/security/) or directly to our security bug bounty site [https://hackerone.com/automattic](https://hackerone.com/automattic).

2. A "Join us" section to invite folks interested in Automattic. Something like this:

## Join us!

Interested in working on awesome open-source code all day? [Join us](https://automattic.com/work-with-us/)!

3. Since we're on GitHub where the plugin needs to be built, maybe there should be a section mentioning the necessary tools to work in this repo, and what to do to build the plugin?

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@pfefferle What do you think?

[pfefferle]

@jeherve would you like to work on that?

[jeherve]

I'm not super familiar with your Grunt config, but I can take a look for sure, at some point in the next few weeks.

[pfefferle]

We do not have to stick with grunt!