docs: add a readme

rockspecs: more detailed description

hglib: commands now can receive input

spec: exit codes are not very useful, check them last Error and output channels usually have readable messages explaining what went wrong, they need to be checked first.

spec: assert(io.open()) to fail early

hglib: have an error message for unexpected data on required channel

hglib: consume command as one argument to runcommand() (allows other arguments)

.luacheckrc: use std=min (in hope of supporting Lua 5.2+ some day)

pipelines: missing dependency for log example

pipelines: also test the new log example