RFC - Update templates to include reminders to first look for duplicates

[ShaunaGordon]

There are probably a dozen or more duplicates for the broken Pong link in the current version (it's fixed in dev and the upcoming release), and more keep coming in. Github, unlike so many others, doesn't have existing issue detection, and while it seems like it should be a given to look for duplicates, clearly a number of people don't think to do it. Having a reminder in the templates might help nudge them to look for existing issues before filing their own.