docs: Add readmes to individual helm charts

[vyzigold]

What this PR does / why we need it:
This PR adds readme file to each chart, so it can be shown on the helm hub. Currently the helm hub tries to find and show the file indefinitely, which doesn't look good: current loki chart page. With the readme files it will look similar to current fluentbit chart

Which issue(s) this PR fixes:
Fixes #1483

[CLAassistant]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

Merging #2233 into master will increase coverage by 0.12%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #2233      +/-   ##
==========================================
+ Coverage   62.04%   62.17%   +0.12%     
==========================================
  Files         156      156              
  Lines       12531    12531              
==========================================
+ Hits         7775     7791      +16     
+ Misses       4147     4130      -17     
- Partials      609      610       +1     

Impacted Files	Coverage Δ
pkg/promtail/targets/filetarget.go	68.67% <0.00%> (-1.81%)	⬇️
pkg/logql/evaluator.go	92.72% <0.00%> (+0.41%)	⬆️
pkg/promtail/targets/tailer.go	78.40% <0.00%> (+2.27%)	⬆️
pkg/promtail/positions/positions.go	60.71% <0.00%> (+13.39%)	⬆️

[owen-d]

wow, thanks for this. The ingress/http basic auth bits are great.