[MRG] Add Gitter info to the README

[mr-eyes]

This PR Fixes #1618 by just adding the Gitter badge to the README and link to it in the Support section.

Ready for review.

[codecov]

Codecov Report

Merging #1658 (272bd7a) into latest (eeb1874) will increase coverage by 7.40%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           latest    #1658      +/-   ##
==========================================
+ Coverage   82.22%   89.63%   +7.40%     
==========================================
  Files         113       86      -27     
  Lines       11704     7990    -3714     
  Branches     1478     1478              
==========================================
- Hits         9624     7162    -2462     
+ Misses       1820      568    -1252     
  Partials      260      260              

Flag	Coverage Δ
python	89.63% <ø> (ø)
rust	?

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Impacted Files	Coverage Δ
src/core/src/signature.rs
src/core/tests/minhash.rs
src/core/src/ffi/nodegraph.rs
src/core/src/ffi/utils.rs
src/core/src/sketch/minhash.rs
src/core/src/ffi/signature.rs
src/core/src/sketch/nodegraph.rs
src/core/src/sketch/hyperloglog/mod.rs
src/core/src/index/search.rs
src/core/src/wasm.rs
... and 17 more

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