(Maint) Clarify that Redmine tickets are mandatory

Prior to this commit, it was unclear if it was required to have
a Redmine ticket before submitting a pull request. Now it is clear
that this is a requirement in order to help us keep track of
submissions.

Also add details about looking for already existing tickets that
are either duplicates or related. This should help with the issue
of having tons of tickets all related to one bigger issue which
cannot be tracked easily.

<hailee@puppetlabs.com>