`prettyprinter-compat-ansi-wl-pprint ` is not quite a drop-in replacement as claimed

[georgefst]

This was noticed in ekmett/ansi-wl-pprint#32 (comment), but I've found some other incompatibilities:

• There's no Pretty a => Pretty (Maybe a) instance, which old code may rely on.
• We have Doc = Doc AnsiStyle, which means that any existing Pretty instances which use basic functions like text no longer compile, since prettyprinter's pretty has to be annotation-polymorphic.

These are not easily solved, so I think the README ought to just be clearer about the limitations.

This has caused issues for some ansi-wl-pprint-dependent packages, such as tmp-postgres.

[georgefst]

This has caused issues for some ansi-wl-pprint-dependent packages, such as tmp-postgres.

jfischoff/tmp-postgres#279

[sjakobi]

• There's no Pretty a => Pretty (Maybe a) instance, which old code may rely on.

That instance actually exists and seems to have existed for quite some time. :|