documentation/style-guide.md - Mention bash portability tip
Created by: totten
If a reader has customized their interactive shell, their system could have small discrepancies in shell notation. It shouldn't matter 95% of the time, and it's not critical to support those folks perfectly. But it is nice. So let's just share the link to heighten awareness.
(Example: I usually do export VAR=value... but in future docs, I'll try
to use VAR=value; export VAR.)
This is following-up on some comments from @ginkgomzd who found the
bash-hackers.org site and shared it on https://github.com/civicrm/civicrm-buildkit/pull/365
