testutil/compose: add splitting to readme

[corverroos]

Add splitting public testnet keys to readme.

category: docs
ticket: #568

[codecov]

Codecov Report

Merging #606 (db5cb44) into main (5c4dd41) will increase coverage by 1.56%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main     #606      +/-   ##
==========================================
+ Coverage   54.66%   56.22%   +1.56%     
==========================================
  Files          96       96              
  Lines        9010     9083      +73     
==========================================
+ Hits         4925     5107     +182     
+ Misses       3378     3236     -142     
- Partials      707      740      +33     

Impacted Files	Coverage Δ
cluster/distvalidator.go	52.38% <0.00%> (-19.05%)	⬇️
testutil/compose/define.go	40.32% <0.00%> (-8.72%)	⬇️
app/app.go	58.20% <0.00%> (-2.50%)	⬇️
core/dutydb/memory.go	77.56% <0.00%> (-1.93%)	⬇️
core/qbft/qbft.go	82.13% <0.00%> (-0.44%)	⬇️
cluster/test_cluster.go	0.00% <0.00%> (ø)
dkg/dkg.go	50.98% <0.00%> (+0.25%)	⬆️
testutil/compose/lock.go	59.13% <0.00%> (+0.38%)	⬆️
cmd/createcluster.go	51.51% <0.00%> (+0.57%)	⬆️
testutil/compose/config.go	83.33% <0.00%> (+0.98%)	⬆️
... and 5 more

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