RELEASE.md: Add PR-based release instructions
RELEASE.md: Add PR-based release instructions

This adds copy-pasteable release instructions that will result in a
release PR rather than an actual release.

This is based on [YARPC Go's RELEASE.md][1].

  [1]: https://github.com/yarpc/yarpc-go/blob/75186108d3da5fc2fad902dd315d15829e66300b/RELEASE.md

thriftrw/thriftrw-go,thriftrw/thriftrw-go,thriftrw/thriftrw-go