added README.md to 19-concurrency/primes/

Author: ramalho

19-concurrency/primes/README.md

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+# Race condition in orignal procs.py
+
+Thanks to reader Michael Albert who noticed the code I published during the Early Release had a race condition in `proc.py`.
+
+If you are curious,
+[this diff](https://github.com/fluentpython/example-code-2e/commit/2c1230579db99738a5e5e6802063bda585f6476d)
+shows the bug and how I fixed it—but note that I later refactored
+the example to delegate parts of `main` to the `start_jobs` and `report` functions.
+
+The problem was that I ended the `while` loop that retrieved the results when the `jobs` queue was empty.
+However, it was possible that the queue was empty but there were still processes working.
+If that happened, one or more results would not be reported.
+I did not notice the problem when I tested my original code,
+but Albert showed that adding a `sleep(1)` call before the `if jobs.empty()` line made the bug occur frequently.
+I adopted one of his solutions: have the `worker` function send back a `PrimeResult` with `n = 0` as a sentinel,
+to let the main loop know that the process had completed, ending the loop when all processes were done.