Update link to release.sh and add link to boilerplate plugin repo

[benlk]

We have docs for Ryan's fork of PMP's release.sh in this repository, but we don't have the actual script in this repo, and we may want to update the script in the future.

Another option would be to make the script have its own repository

[benlk]

No, in the (yet to be created) plugin boilerplate repo. We'll need to update projects/wordpress-plugins/release.sh.md with the link to that repo when that repo is created.

[benlk]

Add ./**/.* to the BLACKLIST variable, so we also exclude dotfiles in subdirectories.

This will prevent dependencies' .git directories from being included in the release zip.

[benlk]

With wp scaffold now available, a plugin boilerplate repo seems redundant. Therefore:

• put release.sh and release.sh.md into their own repo
• replace those files in this repo with text/markdown links to the new file in the new repo, for backwards compatibility where people have linked to those files.