docs: add contributing page

[wesleygrimes]

PR Checklist

Please check if your PR fulfills the following requirements:

• The commit message follows our guidelines: https://github.com/ngrx/platform/blob/master/CONTRIBUTING.md#commit
• Tests for the changes have been added (for bug fixes / features)
• Documentation has been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[x] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Closes #1583

What is the new behavior?

Adds new Contributing page to the NgRx.io website.

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

[ngrxbot]

Preview docs changes for b034d13 at https://previews.ngrx.io/pr1911-b034d13/

[ngrxbot]

Preview docs changes for 6efd730 at https://previews.ngrx.io/pr1911-6efd730/

[ngrxbot]

Preview docs changes for b71c15c at https://previews.ngrx.io/pr1911-b71c15c/

[ngrxbot]

Preview docs changes for 062388b at https://previews.ngrx.io/pr1911-062388b/