The picom project is no longer a fledgling fork needing to justify its
existence. The README should present the user with relevant information
regarding the current project rather than apologies for a 6-year-old
fork of a longer-defunct prior project.
1. Rename README_orig.md to History.md, so it can contain other content
of a historical nature.
2. Move still-relevant historical content from README to History.
3. Add a brief intro blurb to README saying that picom is an X
compositor.
4. Mention Compton in the CONTRIBUTORS section.
5. Point to licensing information from README.
