Haskell-ide-engine: Document that auto-import doesn't work when you don't have a Hoogle database

[masaeedu]

Spent about half an hour goofing around with my hie and vim settings only to realize the hsimport feature doesn't work unless you have a Hoogle database. Could we document this caveat in the README?

[fendor]

We could document that some stuff generally doesnt work if the Hoogle DB is missing, e.g. when no documentation is locally available, Hoogle is asked for documentation.

[samuelpilz]

But there should be a LSP-Warning when the HoogleDB is missing, which should be enough already.

[masaeedu]

I've seen that warning before, but I had no idea what the warning pertained to (as in, why do I need a Hoogle database). What's missing for a new user is the association between use of hsimport and the Hoogle database (either in docs or in the warning).

[fendor]

Adding a warning and documentation seems very sensible to me, I am in favour of doing both