Vuepress: Create CONTRIBUTING.md

[sobolevn]

Feature request

We need to create a CONTRIBUTING.md guide.
Original idea from #1745 and #1741

What problem does this feature solve?

It allows new contributors to understand the process of creating new PRs to the repo.
Ideally, it covers topics like:

1. Forking and branching, including branch names and commit messages
2. Project and mono-repo layout
3. Code quality standards and agreements
4. CI process and how to fix it
5. Common tips that we know about

Are you willing to work on this yourself?

I would better ask for help of someone who works on this repo for a longer period of time.
And knows all the conventions and tools.

[sarahdayan]

Hey! @ulivz created one already, let's finish it :) https://github.com/vuejs/vuepress/pull/1739

[f3ltron]

I am closing this one because the #1739 is merged feel free to re open it if you think it should not be close