Summary:
Stop mentioning the individual directories.
Added README.md into some directories that are more important than
others.

Test Plan: Jenkins

Reviewers: #engineering, zasgar

Reviewed By: #engineering, zasgar

Differential Revision: https://phab.corp.pixielabs.ai/D2509

GitOrigin-RevId: 1b98310fb9882fa7d1a320179651d8f0286a67df