Write a getting started guide about events, invoke and call

[cescoffier]

Fix #399

@mkouba FYI.

[mkouba]

Looks good ;-).

[codecov]

Codecov Report

Merging #400 (60cb431) into master (7f58314) will increase coverage by 0.02%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master     #400      +/-   ##
============================================
+ Coverage     89.38%   89.40%   +0.02%     
  Complexity     2721     2721              
============================================
  Files           363      363              
  Lines         10520    10524       +4     
  Branches       1304     1308       +4     
============================================
+ Hits           9403     9409       +6     
  Misses          579      579              
+ Partials        538      536       -2     

Impacted Files	Coverage Δ	Complexity Δ
...ava/io/smallrye/mutiny/helpers/HalfSerializer.java	77.77% <0.00%> (-11.12%)	8.00% <0.00%> (-1.00%)
.../io/smallrye/mutiny/helpers/queues/DrainUtils.java	77.77% <0.00%> (-8.89%)	12.00% <0.00%> (-2.00%)
...subscription/SwitchableSubscriptionSubscriber.java	95.08% <0.00%> (-2.46%)	45.00% <0.00%> (-1.00%)
...o/smallrye/mutiny/operators/UniAndCombination.java	84.88% <0.00%> (-1.17%)	4.00% <0.00%> (ø%)
...lrye/mutiny/subscription/SerializedSubscriber.java	90.56% <0.00%> (-0.95%)	28.00% <0.00%> (-2.00%)
...mallrye/mutiny/operators/multi/MultiFlatMapOp.java	85.71% <0.00%> (-0.32%)	3.00% <0.00%> (ø%)
...ain/java/io/smallrye/mutiny/groups/MultiMerge.java	100.00% <0.00%> (ø)	6.00% <0.00%> (ø%)
...in/java/io/smallrye/mutiny/groups/MultiConcat.java	100.00% <0.00%> (ø)	4.00% <0.00%> (ø%)
...ava/io/smallrye/mutiny/operators/MultiCombine.java	100.00% <0.00%> (ø)	4.00% <0.00%> (+2.00%)
...o/smallrye/mutiny/groups/MultiItemCombination.java	100.00% <0.00%> (ø)	10.00% <0.00%> (ø%)
... and 7 more