docs: hint about easy overloading of autodetected engine with self selected preference

[salzig]

Was surprised that it's possible to overload the auto detection using just a variable. Hope it's ok to just state it at this prominent location in the readme.

[salzig]

  1) Failure:
TestExecJS#test_timers_are_undefined [test/test_execjs.rb:289]:
Expected false to be truthy.
  2) Failure:
TestExecJS#test_console_is_undefined [test/test_execjs.rb:284]:
Expected false to be truthy.

I'm pretty sure i didn't introduce this error by a readme change :)

[yagarea]

I vote for merging of this PR. This is very important and hard to find information