Clarify tagging process and ressurect tag.sh for use with nonrelease builds

Tagging process differs between a release build and others. Docs now suggest using ``tag.sh`` to speed up tagging for all non release builds and doing it manually for releases. This allows the builder to combine the changelogs for all of the builds leading up to the release, similar to a git squash. The resulting changelogs are cleaner and the build process is fast for the majority of the builds.