Split README.md into classical README and CONTRUBITING

[anton-trunov]

The following sections should be moved from README into CONTRIBUTING:

• https://github.com/Zilliqa/scilla/blob/master/README.md#running-the-testsuite
• https://github.com/Zilliqa/scilla/blob/master/README.md#formatting-and-linting-the-codebase

Looks like some parts of INSTALL.md should also be moved to CONTRUBITING.