Lesson repositories should default to a README as their "main" field

[SheaBelsky]

With the loss of the ability to point to a testing file in #51 , we should have all repos point to a human-readable README file (with instructions on where to begin) if they cannot immediately start coding. We need to do this for our Array Methods, Data Types, Functions and Scopes, and Promises courses, but the courses on React Hooks and Testing already have descriptive "starting" points where this should not be necessary.