Add man page

[mattmahn]

I've written a man page for git-standup in AsciiDoc, just like all the other Git man pages. This completes #68; all of the following open the man page:

• man -l ./git-standup.1.gz
• git help standup
• git standup --help man git standup
• man git-standup

I've also done my best to abide by the style guide in man-pages(7).

Note: you'll need the asciidoc package for the a2x command to convert the file.

[z0rc]

That's nice, but probably a2x dependency is too heavy just for generating man page, while rest of the project is bash script. I recommend maintaining regular man page. It isn't that easy formatted like asciidoc, but still doable. Check out git-extras pages.

[mattmahn]

I thought a2x might be a bit much.

I've hand-wrote it now, but I seem to have written such that it doesn't disaply right on macOS/BSD. So I guess I'll be fixing that later...

[guillemglez]

How come this was never merged? Would it be worth trying to rebase these changes and resubmit?