Add info about Table::addAssociations()
Document firstOrFail and collection methods.

Document the new method and the ability to use collection methods on
query objects.

Refs #2079
update addAssociations()
Merge pull request #2081 from cakephp/3.0-add-associations

Add info about Table::addAssociations()
Merge pull request #2082 from cakephp/issue-2079

Document firstOrFail and collection methods.