Add tips and tricks for building

[rstanuwijaya]

I think we need a better documentation about some tips and tricks of building Zen.

My favorite command is:

$ git -C engine/ reset --hard && npm run import && npm run build:ui

to reapply patches without needing of full rebuild.

And perhaps:

$ npm run surfer update

to supposedly update to the latest Firefox version (I tried to run this but it somehow says no new firefox version is found? So I ended up removing the entire engine/ directory and rerun everything from npm run init)