doc: address use of profanity in code of conduct

PR-URL: #3827
Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Julien Gilli <jgilli@fastmail.fm>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
Reviewed-By: Alexis Campailla <alexis@janeasystems.com>

Author: jasnell

CODE_OF_CONDUCT.md

@@ -10,6 +10,8 @@ CoC](http://www.rust-lang.org/conduct.html).
   might detract from a friendly, safe and welcoming environment for
   all.
 * Please be kind and courteous. There's no need to be mean or rude.
+* Respect that some individuals and cultures consider the casual use of
+  profanity offensive and off-putting.
 * Respect that people have differences of opinion and that every
   design or implementation choice carries a trade-off and numerous
   costs. There is seldom a right answer.