Make CONTRIBUTING.md, README.md, and PR Template consistent

[stephendolan]

It was a bit confusing to figure out what's necessary for contributing to the progress (at least, for a first-time contributor).

First, I checked CONTRIBUTING.md, which has:

## Submitting a Pull Request
1. [Fork][fork] the [official repository][repo].
2. [Create a topic branch.][branch]
3. Implement your feature or bug fix.
4. Add, commit, and push your changes.
5. [Submit a pull request.][pr]

But, upon creating a pull request, I noticed these tasks:

## Checklist
* [ ] - An issue already exists detailing the issue/or feature request that this PR fixes
* [ ] - All specs are formatted with `crystal tool format spec src`
* [ ] - Inline documentation has been added and/or updated
* [ ] - Lucky builds on docker with `./script/setup`
* [ ] - All builds and specs pass on docker with `./script/test`

This was the first mention of Docker I'd encountered, so I went digging through the README and found:

## Contributing

1. Fork it ( https://github.com/luckyframework/web/fork )
1. Create your feature branch (git checkout -b my-new-feature)
1. Install docker and docker-compose: https://docs.docker.com/compose/install/
1. Run `script/setup` to build the Docker containers with everything you need.
1. Make your changes
1. Make sure specs pass: `script/test`.
1. Add a note to the CHANGELOG
1. Commit your changes (git commit -am 'Add some feature')
1. Push to the branch (git push origin my-new-feature)
1. Create a new Pull Request

> Run specific tests with `script/test <path_to_spec>`

### Lucky to have you!

We love all of the community members that have put in hard work to make Lucky better.
If you're one of those people, we want to give you a t-shirt!

To get a shirt, we ask that you have made a significant contribution to Lucky.
This includes things like submitting PRs with bug fixes and feature implementations, helping other members
work through problems, and deploying real world applications using Lucky!

To claim your shirt, [fill in this form](https://forms.gle/w3PJ4pww8WDAuJov5).

I'd recommend:

• Moving the README "Contributing" content to CONTRIBUTING
• Linking to CONTRIBUTING for the README "Contributing" content
• Moving the PR Template checklist to CONTRIBUTING
• Simplifying the PR template checklist to one "I have adhered to all rules in CONTRIBUTING.md" or something

Happy to tackle this PR myself, but want to make sure the group is okay with the change. I know it would have helped me a bunch to have a single place to look for everything I need to contribute to the project!

[stephendolan]

Per #1206 , we also may want to remove the "Add a note to the CHANGELOG" requirement in the README contributing section.

[bwks]

I agree with this, I was a bit confused when I first submitted a PR.