Add a note on "thread" safety

[alecmev]

Given that result is a global var, I believe it's possible for the output to get corrupted (call 2 overwriting call 1's result), if MTRC gets invoked multiple times simultaneously. It appears, though, that both MTRC and Marked are fully synchronous, so there's no opportunity for multi-tasking to kick in between exec call and its completion, but I had to examine Marked to be sure of that. A quick comment on this somewhere in the code or in the readme would be helpful. Thanks.