Adds a warning of non thread safety for withConstantNow and ConstantNowTestListener in readme. (#1720)
Adds a warning of non thread safety for withConstantNow and ConstantNowTestListener in readme. (#1720)

withConstantNow and ConstantNowTestListener are not thread safe, this
has been already documented in java doc, but not readme file.
Chances of people reading java doc is very less compare to Readme file.
Adding it to readme file will increase chances of people knowing about
its non thread safe nature beforehand.
sksamuel/ktest