Minifies the current development guide by
a significant amount. Also, removes system requirements
as no credible source for them. Removes dependency
specific instructions and adds links to instructions
to install them.
Fixes#341.
This gives an explicit and hopefully-complete set of steps to run
the app from source on a Debian-based system: worked for me on my
Debian "stretch" (the recent release) laptop, though no guarantees
there wasn't some previously-installed package I was relying on.
Also make the generic prerequisites section include everything we
know about, including in particular everything mentioned in the
Debian/Ubuntu section. Hopefully this is helpful for people working
on other OSes too -- though specific instructions for those would be
very helpful!
