Update the CONTRIBUTING.md & GETTING_STARTED.md guides

[obensource]

Description

Essentially, we don't have a contributing guide, and should extrapolate our getting started guide. We need clear documentation that enables participants to get involved in all major aspects of this initiative:

• How to join the i18n WG, and get involved in the initiative.
• How to work on the i18n repo to localize content coming from Crowdin.
• How to join Crowdin and localize Node.js resources as a translator.
• How to get involved in contributing to Intl in Node.js.
• How to gather volunteers to help out with all aspects of this initiative.

This, along with steady contributions from the i18n WG, will foster a lot of progress for Node.js i18n. 🎉

[srl295]

modify #210 per today;'s discussion

[RichardLitt]

Note from today's call: Neither I nor @zeke have the time to do this right now, but we'd love to see this move forward if anyone wants to take a stab at a first draft!

[alexandrtovmach]

Closed in favor #266 #270