Add FAQ about source of randomness (/dev/urandom versus /dev/random).

pep-0506.txt

@@ -338,6 +338,15 @@ Frequently Asked Questions
      module, instead it will be given in the documentation as a recipe (à la
      the recipes in the ``itertools`` module) [30]_.
 
+* Q: Will ``secrets`` use /dev/random (which blocks) or /dev/urandom (which
+     doesn't block) on Linux? What about other platforms?
+
+  A: ``secrets`` will be based on ``os.urandom`` and ``random.SystemRandom``,
+     which are interfaces to your operating system's best source of
+     cryptographic randomness. On Linux, that may be ``/dev/urandom`` [31]_,
+     on Windows it may be ``CryptGenRandom()``, but see the documentation
+     and/or source code for the detailed implementation details.
+
 
 References
 ==========
@@ -412,6 +421,9 @@ References
 
 .. [30] https://mail.python.org/pipermail/python-ideas/2015-September/036488.html
 
+.. [31] http://sockpuppet.org/blog/2014/02/25/safely-generate-random-numbers/
+        http://www.2uo.de/myths-about-urandom/
+
 
 Copyright
 =========