Add notes on futex linux syscall.

This might allow us to do lock free, conditional waits in our stealing loop.
4 jobs from master in 4 minutes 9 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #3276
build_cmake

00:46

 
  Test
passed #3277
run_tests

00:44

 
  Sanitizer
passed #3279
run_address_sanitizer

01:38

passed #3278
run_thread_sanitizer

00:59