Add template for adding new tips and instructions in readme

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New tips can be added in the tips folder, but README does not feature instructions on how to do it and the tips have no metadata section at the top. We should do something similar as in the MTG internal blog, have some instructions about how to add new tips and have a template with a couple metadata fields at the top so when rendered in https://mtg.github.io/pymtg/tips/tips.html we can display the date of the tips.

A simpler way to do it would be to simply add date to the titles, but this would be less flexible.