Add permalinks step to Quick Setup instructions

The `/wp-json/` endpoint is not accessible when following the Quick
Setup instructions, because the rewrite rule for the endpoint is not
triggered with the default permalink structure.

Including a step in the setup instructions to explicitly set a
compatible permalink structure should reduce confusion.
Clarify API endpoint with Default permalinks

Lines wrapped to 80 chars per PR feedback
Merge pull request #183 from kadamwhite/clarify-setup-instructions

Add permalink settings step to Quick Setup instructions