Enhance README for expectAssertion
Enhance README for expectAssertion

- As per [this
issue](https://github.com/workmanw/ember-qunit-assert-helpers/issues/11)
it is not very clear from the documentation how is expectAssertion
expected to use.
- This PR adds explicit statement that the matcher should be regular
expression.
- Also adds more explicit example to show the possibilities when
matching regular expressions.

workmanw/ember-qunit-assert-helpers,workmanw/ember-qunit-assert-helpers