Document how to create a common baseline file for multi module gradle projects

[arturbosch]

Closes #2100

[codecov-io]

Codecov Report

Merging #2140 into master will decrease coverage by 0.69%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             master   #2140     +/-   ##
==========================================
- Coverage      81.2%   80.5%   -0.7%     
+ Complexity     2064    2015     -49     
==========================================
  Files           340     336      -4     
  Lines          5910    5802    -108     
  Branches       1076    1064     -12     
==========================================
- Hits           4799    4671    -128     
- Misses          535     564     +29     
+ Partials        576     567      -9

Impacted Files	Coverage Δ	Complexity Δ
...ain/kotlin/io/gitlab/arturbosch/detekt/cli/Main.kt	0% <0%> (-50%)	0% <0%> (-7%)
...itlab/arturbosch/detekt/cli/out/XmlOutputReport.kt	68.57% <0%> (-31.43%)	6% <0%> (-3%)
...ab/arturbosch/detekt/cli/runners/ConfigExporter.kt	0% <0%> (-20%)	0% <0%> (-1%)
.../kotlin/io/gitlab/arturbosch/detekt/cli/CliArgs.kt	92.59% <0%> (-7.41%)	6% <0%> (ø)
...in/io/gitlab/arturbosch/detekt/api/SplitPattern.kt	78.94% <0%> (-6.24%)	17% <0%> (ø)
...lab/arturbosch/detekt/formatting/FormattingRule.kt	69.23% <0%> (-4.85%)	8% <0%> (-1%)
...kotlin/io/gitlab/arturbosch/detekt/api/Location.kt	77.77% <0%> (-3.71%)	4% <0%> (ø)
...gitlab/arturbosch/detekt/cli/runners/AstPrinter.kt	74.28% <0%> (-2.86%)	0% <0%> (-1%)
...ch/detekt/api/internal/ValidatableConfiguration.kt	85.71% <0%> (-2.86%)	0% <0%> (ø)
...otlin/io/gitlab/arturbosch/detekt/api/CodeSmell.kt	58.13% <0%> (-2.33%)	11% <0%> (ø)
... and 19 more

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[schalkms]

Cool thanks!

[adrianczuczka]

Can we move the entire task to the subprojects block? It did not work correctly for me without doing that.

[arturbosch]

It should work on top level like in our build file. Inside subprojects every sub-project would get a baseline task hm :/

[arturbosch]

@adrianczuczka thanks for the suggestions, I've incorporated them and added a detektProjectBaseline task for detekt too.