document build.ninja syntax

[readmemyrights]

Samurai right now only ships with samu.1,
which only documents command usage
If someone wanted to read about the input it accepts
one would have to either:

• Read the documentation online, or
• install ninja and read its manual instead

For people who'd like offline full documentation without installing
additional software,
would it be possible to include ninja's manual.asciidoc
or write a new reference for the manual format?

I would be open to writing the necessary documentation
with the help of someone more experienced
with making .ninja files.

[michaelforney]

I think that's a good idea. If someone wants to write up ninja.5, I'm happy to review it.