LeakSanitizer has encountered a fatal error.

[mrcnski]

We're seeing this error on the completion of cargo test on Linux. It seems to happen for every project, but only on Linux and not OSX. We've seen this same error in CI and docker so far.

==9196==LeakSanitizer has encountered a fatal error.
==9196==HINT: For debugging, try setting environment variable LSAN_OPTIONS=verbosity=1:log_threads=1
==9196==HINT: LeakSanitizer does not work under ptrace (strace, gdb, etc)

https://travis-ci.org/maidsafe/parsec/jobs/415533268#L885

[mrcnski]

This problem has been reported a couple times here, with some solutions proposed.

Here, we just need to add a note to the Caveats section in the README about how to run the sanitizer in Docker and Travis.