Add some more best practices that I've seen come up on reviews quite often.

Add a section on not returning null since the canonical data often cause
confusion about this.

Add a section on using ExpectedException since this was something we agreed
a while ago but it's not documented anywhere.

We have decided that every exercise with diffulty less than 3 should
reference the hello world tutorial to make it easier for people to solve
those exercises. This has now been added to our policies doc.

We also now reference those policies in the contributing doc in the section
for adding new exercises.

Add reference to section about adding a hints file to all exercises
with difficulty 5.

* Add hints to exercises without starter implementation

As exercises with difficulty 5 are the first exercises without starter
implementation, they should have a hints file explaining how to fix the
compiler errors that occur when an exercise doesn't have any starter
implementation.

This should be detailed in the policies doc.

* flatten-array: regenerate README

Also update description for how to generate readme as it was lacking
detail.