[docs] warn that docker installation instructions are contrib/

[mistercrunch]

No description provided.

[codecov-io]

Codecov Report

Merging #6925 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #6925      +/-   ##
==========================================
+ Coverage   63.96%   63.99%   +0.03%     
==========================================
  Files         422      423       +1     
  Lines       20524    20555      +31     
  Branches     2230     2235       +5     
==========================================
+ Hits        13128    13154      +26     
- Misses       7264     7269       +5     
  Partials      132      132

Impacted Files	Coverage Δ
superset/assets/src/chart/ChartRenderer.jsx	7.24% <0%> (-0.57%)	⬇️
...uperset/assets/src/datasource/DatasourceEditor.jsx	61.84% <0%> (ø)	⬆️
superset/assets/src/chart/transformBigNumber.js	100% <0%> (ø)
superset/models/core.py	83.61% <0%> (+0.05%)	⬆️
superset/connectors/sqla/models.py	81.8% <0%> (+0.09%)	⬆️
superset/db_engine_specs.py	56.75% <0%> (+0.44%)	⬆️

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