Generate checksum files for archives released on GitHub

[bbarker]

This might be added to the archive building script (located where?). Supported checksums should include:

• md5sum
• sha256sum (used by NixPkgs)
• others?

I'll be happy to do this at some point, if someone doesn't get to it first.

[smarter]

I think just sha256sum should be enough

[allanrenucci]

We use sbt-pack to generate the archive on GitHub:

sbt dist-bootstrapped/pack-archive

I wanted to automate the process with the CI and the GitHub Release Plugin for Drone. The plugin can generate the checksum for you.