Summary: * Add a Table of Contents to README * Link CONTRIBUTING section to CONTRIBUTING.md file * Link License section to LICENSE file * Change `Sonar` references to `Flipper` in CONTRIBUTING.md  Pull Request resolved: https://github.com/facebook/flipper/pull/222 Reviewed By: danielbuechele Differential Revision: D9282868 Pulled By: passy fbshipit-source-id: 2f1ff9b598b8a0afdde9039e5a2c4a157c48ed9c
fbshipit-source-id: b6fc29740c6875d2e78953b8a7123890a67930f2 Co-authored-by: Sebastian McKenzie <sebmck@fb.com> Co-authored-by: John Knox <jknox@fb.com> Co-authored-by: Emil Sjölander <emilsj@fb.com> Co-authored-by: Pritesh Nandgaonkar <prit91@fb.com>