Document specifying a custom JIRA issue key regex

JIRA has a UI to help specify a custom issue key format.

Stash can allow JIRA integration with custom issue key formats by editing the <STASH_INSTALL>/bin/setenv.sh or <STASH_INSTALL>/bin/setenv.bat depending upon the OS.
To do this, pass -Dstash.jira.key.pattern=<some different regex> parameter to the JVM_SUPPORT_RECOMMENDED_ARGS property, such as:

JVM_SUPPORT_RECOMMENDED_ARGS="-Dstash.jira.key.pattern=(<Some different regex>)"

It would be good to make this configurable via the UI instead.