Add community forum to README.md

[simonswine]

What this PR does / why we need it:

Add the community forum after seeing the blog post: https://grafana.com/blog/2020/11/19/loki-oss-support-moving-from-slack-to-grafana.com-community-forums/

[codecov-io]

Codecov Report

Merging #2969 (67fe4c5) into master (a62f81f) will increase coverage by 0.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2969      +/-   ##
==========================================
+ Coverage   61.75%   61.85%   +0.09%     
==========================================
  Files         182      182              
  Lines       14866    14866              
==========================================
+ Hits         9181     9195      +14     
+ Misses       4838     4824      -14     
  Partials      847      847              

Impacted Files	Coverage Δ
pkg/querier/queryrange/downstreamer.go	95.29% <0.00%> (-2.36%)	⬇️
pkg/promtail/targets/file/tailer.go	71.42% <0.00%> (+1.90%)	⬆️
pkg/promtail/targets/file/filetarget.go	66.19% <0.00%> (+2.11%)	⬆️
pkg/promtail/positions/positions.go	58.51% <0.00%> (+11.70%)	⬆️

[slim-bean]

thanks @simonswine