1. These days we delete the [Unreleased] tag during rcs.
2. Make sure we test the release build process during rc1, since I
screwed that up last release.
3. Add a section on rc2, etc.
4. Do final release via a github PR, since I screwed that up on the
prior release.
5. Update `tools/build-release.sh` and instructions to show that we now
make a reproducible build for Ubuntu 18.04 x86-64.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>