Issue template for merchants

A lot of the activity on this repo is from merchants asking to add their business to our merchants list. I created a GitLab issue template to make sure business owners submit all the information necessary to be listed. This will make faster and much easier to verify the legitimacy of the merchant, but will also give as a quick overview of how they accept Monero and will help us categorize it. I believe we could make bigger use of issue templates, but let's start slow :)

I also edited the disclaimer in the merchant page, that now invites merchants to use the template when requesting to be added to the website. I added a new key (intro3) instead of using intro2, because it will make the work easier for translators.

I created a test repo which icludes this template to show how it would work. Open an issue on the test repo and select the Merchant template. Will work the same way in this repo.

Feel free to suggest better wording for the template, i'm aware can be improved.