Make more generalised documentation

[sloorush]

Make docs for new readers.

Docs could consist of:

• [ ] How to add new tests
• [ ] How to modify old tests

  Excerpt from Karta's email: For example, suppose we modify a primitive to add a new algorithm which gets invoked under some criteria. This is a new switch point, and it would be nice to use some standardised routines that you have written to QA this new switch point.

• [ ] How the test suite is written (this is kind of there in the decision docs)
• [ ] How to add and modify data and types

To be continued...