ARROW-4756: Update readme for triggering docker builds

[pravindra]

No description provided.

[codecov-io]

Codecov Report

Merging #3802 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3802      +/-   ##
==========================================
- Coverage   87.79%   87.77%   -0.02%     
==========================================
  Files         708      708              
  Lines       86591    86591              
  Branches     1233     1233              
==========================================
- Hits        76019    76008      -11     
- Misses      10459    10466       +7     
- Partials      113      117       +4

Impacted Files	Coverage Δ
go/arrow/math/int64_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/memory/memory_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/math/float64_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/math/uint64_avx2_amd64.go	0% <0%> (-100%)	⬇️
go/arrow/memory/memory_amd64.go	28.57% <0%> (-14.29%)	⬇️
go/arrow/math/math_amd64.go	31.57% <0%> (-5.27%)	⬇️
cpp/src/arrow/csv/column-builder.cc	95.39% <0%> (-1.98%)	⬇️
go/arrow/math/float64_amd64.go	33.33% <0%> (ø)	⬆️
go/arrow/math/int64_amd64.go	33.33% <0%> (ø)	⬆️
go/arrow/math/uint64_amd64.go	33.33% <0%> (ø)	⬆️
... and 5 more

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[xhochy]

+1, LGTM. Thanks for documenting this!

[pravindra]

thanks @xhochy