Update README: HLS is no longer in *very* early stage

[Anrock]

Given relative feature-parity with HIE and #473 this line is no longer true, I believe.

[ndmitchell]

Yep, its relatively mature as things go now!

[ndmitchell]

GHC 8.10.2 CI is failing with:

Error: While constructing the build
plan, the following exceptions were
encountered:

In the dependencies
for brittany-0.12.1.1:
    data-tree-print needed, but the
                    stack configuration
                    has no specified
                    version  (latest
                    matching version
                    is 0.1.0.2)
needed due
to haskell-language-server-0.5.0.0
    -> brittany-0.12.1.1

Going to guess that isn't due to a README change, so merged anyway. But would be good to have that fixed.