Add documentation for acbuild script

[cdrage]

@dgonyeo Is it possible to add this to the acbuild script --help command too, or would it be too much? (since acbuild has no man pages)

[cgonyeo]

Not sure, it would make it the largest man page.

Maybe including a link to the GitHub documentation in the help page would help at the very least.

acbuild does also have the ability to generate man pages, but the information is the same as on the help pages.

[cdrage]

@dgonyeo ah okay, yeah, adding a link to the github documentation would be a good half-way point for this.

[cdrage]

@jonboulle quick review perhaps? :)

[jonboulle]

lgtm, thanks