docs: add "Contributing" section

[iuioiua]

This just copies the main repo, which does the job.

[lucacasonato]

Can you add a section to the manual about "Contributing to fresh" with instructions on tests, fmt, and lint?

[deer]

I was thinking of working on #657, but then I saw this. Please close the issue once this is merged. I'd also suggest the link in README.md be changed to https://deno.com/manual/references/contributing#submitting-a-pr-to-fresh, so that people are taken to exactly the right place.

There's also the two closed #465 and #658. Before I delete my branch that introduces a CONTRIBUTING.md file (because what you have here seems like the preferred approach, I'd still recommend some sort of tips section. I had...

- [Conventional commits](https://www.conventionalcommits.org/en/v1.0.0-beta.2/#summary)
  are great when titling your PR.
- Tests, documentation, and examples always help to provide extra context.
- You'll likely need to install
  [deno-puppeteer](https://github.com/lucacasonato/deno-puppeteer#installation)
  in order to run the tests.

[iuioiua]

I was thinking of working on #657, but then I saw this. Please close the issue once this is merged. I'd also suggest the link in README.md be changed to https://deno.com/manual/references/contributing#submitting-a-pr-to-fresh, so that people are taken to exactly the right place.

Done and done.

• Conventional commits are great when titling your PR.

Good idea. But this repo isn't the place to do that. I instead created an issue with this suggestion in the manual repo (here).

• Tests, documentation, and examples always help to provide extra context.

Maybe let's get this PR landed first. We can explore how we can improve instructions in an issue or PR after.

• You'll likely need to install deno-puppeteer in order to run the tests.

I believe this is done anyway upon running deno task test

Ok, I'll add that in.

[iuioiua]

This PR is ready for review.

[deer]

https://github.com/denoland/manual/blob/main/references/contributing.md?plain=1#L165
What is the - this doing at the end here? 🧐

[iuioiua]

Doh! Will fix 🥲

[iuioiua]

@marvinhagemeister, PTAL. I'll fix the typo mentioned in #1233 (comment) independently.

[deer]

@marvinhagemeister Looks like this was inadvertently closed when manual#681 was merged.