Add a note on "thread" safety

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.

Read the README and the MTRC implementation around the global result and exec call. Confirm the synchronous behavior described in the issue, then add a brief note explaining the thread-safety implication in the README or nearby code; done means the concern and current behavior are documented.

Written by the indexing model from the issue text.