MD structures across weeks/modules are inconsistent

[adamblanchard]

Some .md files don't follow the same structure as others.

I haven't spotted any major issues so far other than inconsistency, hard to follow, some good sections in some are missing in others.

I propose we put a template together for each, so when we review the modules we can fix them as best we can to towards matching the template.

Outcomes of this task: A template .md for each of the following:

1. readme.md (module dir)
2. readme.md (weekX dir)
3. homework.md
4. lesson-plan.md
5. preparation.md