message: Add readme file to tests project
diff: +## Botkit Core Tests
  +
  +This directory contains code and data used to test Botkit.
  +
  +For a detailed guide on how to write tests in this directory, see the guide on writing tests.
  +
  +On how to run tests in this directory, see the contributing guide.
  +
  +
  +## Test Directories
  +
  +| Directory | Runs on CI | Purpose |
  +|------------------|----------------------------|----------|
  +| facebook | npm run test:facebook | |
  