Document procedure to build from source

[bignose-debian]

Many of the files in the VCS repository are actually build artefacts: the JavaScript library is evidently generated from a CoffeScript source, for example.

Please document which files are mechanically generated, and the full procedure for generating them from source. The goal is to describe what a recipient needs to do to build from (maybe customised) source, the same way the maintainers do it.