Refactor to improve docs

[sibelius]

Let's follow react-navigation#docs docs structure

[lucasbento]

Basically the idea of this issue is to have a docs folder with .md files that are linked on the README.md under a Table of Contents section.

cc @idkjs.

[felippepuhle]

How can we start this? Maybe we should point the topics, like a 'step by step'?

[lucasbento]

For now we can just put the sections of README into separated .md files, later on we can improve the content.

[danielfttorres]

@felippepuhle I think we can follow the checklist below:

• [x] Clear and improve Readme.md
• [x] Create docs path following react-navigation#docs
• [x] Move and improve existent docs from Readme.md to docs path
• [x] Create ISSUE_TEMPLATE.md and PULL_REQUEST_TEMPLATE.md

I'm working on this to keep improving the documentation of this awesome project. I will send the PR soon.