Add contribution guide

CONTRIBUTING.md

+## Adding New Topics
+
+1. To propose a new topic, open an issue on this Repo
+1. If your topic is approved (it probably will be), @rf9 will contact you to work out a date
+1. Write your slides in Markdown and submit them to this repo -- put all your content in its own folder
+1. In your merge request, be sure to update the top-level README.md with your topic and date
+1. Also update `git-it-together.ics` to include your topic and date
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