Tidy up docs, READMEs and CONTRIBUTING across repos

[fryorcraken]

Problem

There are docs remaining in various repos README's and /doc folder, making it duplicate with the docs website: docs.waku.org.

Docs should be easy to find from the GitHub repos, and outdated docs should be avoided.

Also, there is no CONTRIBUTING.md in some repos.

Proposed Solution

1. Make the http://docs.waku.org/ the doc of reference.
2. Only have README.md with rich content in the root folder of each repo, and make all other sub-README.md files to refer the appropriate http://docs.waku.org/ point.
3. Keep the same philosophy across all repos.

Even the "main" README.md at point 2. could refer to the http://docs.waku.org/ at the end, indicating where the user can find further information about Waku.

4. Re-instante contribute page and point to it from repo's READMEs: 590b7d8