specify somewhere that reproduce only works with libfuzzer

[aureliendavid]

Hi and thanks for a great tool,

This is not really an issue but I'd like to suggest specifying somewhere that the reproduce script will only work when fuzzers are built with the libfuzzer engine, since libfuzzer arguments are automatically passed to the binary (-rss_limit_mb=2560 -timeout=25)

There was some discussion about it in this issue a while ago: #4497 (comment)

Maybe writing a note about it in https://github.com/google/oss-fuzz/blob/masterdocs/advanced-topics/reproducing.md

Even better would be to detect the fuzzing engine when running reproduce and either emitting a warning or just running the binary without libfuzzer arguments if it detects something other than libfuzzer?

It could save some testers some time debugging the wrong binary.

Thanks again.