Add note on atomic sources.

4 jobs from master in 3 minutes 43 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #2920
build_cmake

00:43

 
  Test
passed #2921
run_tests

00:42

 
  Sanitizer
passed #2923
run_address_sanitizer

01:20

passed #2922
run_thread_sanitizer

00:55