create section for proposing, discussing and creating/extending/updating Training Missions

[kmolo]

Create instructions for how to:

• create new training missions
• fork & return PRs to update/extend existing training missions

Would be nice to have somewhere -- either as GitHub issue or Discourse category -- to propose new/updated training mission, receive feedback, and then submit implementation of training mission for peer review.

Potentially, this process could be split into two:

• proposing/creating content
• implementing the interactive training mission for the website

...the content creation should be made as simple as possible, so that lots of it gets created. Then regular creators can be encouraged to cross over into implementing too, or just establishing partnerships with contributors who knock out the implementations.