Makefile: some speedups:

 * lint-free dir isn't a phony task so things that depend on it don't
   have to re-run
 * extract assets/insistent_cat.js to the variable OUT_FILE
 * create a separate target for OUT_FILE so make can bypass it if it's
   up-to-date
 * use `touch` instead of `cp` for marking files as lint-free
 * use $(x:y=z) instead of $(patsubst)
README: add Make section