Prepare initial release

[polytypic]

This PR:

• Adds CHANGES.md with notes for the initial version
• Adds HACKING.md with some minimal instructions on formatting and release instructions for maintainers
• Adds update-gh-pages-for-tag script which is a script developed originally for kcas (and then used in many other repositories under ocaml-multicore) to help maintain documentation for multiple versions of the library on gh-pages
• Updates the link to API reference in the README.md to point directly to the Backoff module page