Install instructions don't build css and javascript by default

[csakoda]

I just installed pastedown (love it, thanks for the great software), and stumbled around a bit during the install.

The README.md (https://github.com/cespare/pastedown/blob/master/README.md) says to install the pre-reqs and run make. I did that, launched up the site, only to find it bare of css and javascript. After poking around in the Makefile I figured out how to generate the missing files, but why not build them automatically?

Thanks again

[Caleb Spare]

Hey, thanks for the feedback and the PR. See comments over there.