More thoroughly document what is (and isn't) allowed in the README

This patch fleshes out some more details about what `singletons`
can and can't do in its `README`. The key changes are:

1. There is a new "Promotion and partial application" section that
   explains what defunctionalization is in some amount of detail.
   There is also a new subsection that explains the limitations of
   the `genDefunSymbols` function that were observed in #429.
2. The "Supported Haskell constructs" section has received some more
   love. Some Haskell features were inaccurately characterized
   (e.g., pattern signatures are really only partially supported),
   so I also reorganized some of the bullet points. I have also added
   a new bullet point for `ScopedTypeVariables` under the
   "Little to no support" section, as #433 reveals that promoting
   functions that rely on the behavior of `ScopedTypeVariables` is
   terribly fragile (and not easy to fix).
3. Lots of little formatting and grammar fixes to make the prose in
   the `README` flow better.

Note that this patch does _not_ fix either of #429 or #433—it just
documents the rather unsatisfying current state of affairs.