Teach the Makefile to build a TAGS file for GNU Emacs

This reintroduces the special rules to create TAGS for our game that
we used to have in the old Tupfile when we were using that build
system instead of GNU Make.
Create a ‘make clean’ rule

This is useful for having a clean slate from which to test existing
features or begin implementing new bugs.
README: Clarify the title of the game
Merge branch 'ejmr/extra-makefile-targets'

* ejmr/extra-makefile-targets:
  README: Clarify the title of the game
  Create a ‘make clean’ rule
  Teach the Makefile to build a TAGS file for GNU Emacs