Emphasize default method for cy.route() is GET

[jennifer-shehane]

cypress-io/cypress#567

Just a side-mention that the failure to match POST should probably be included in the intro-level documentation as a note/gotcha. As a Cypress newbie I got bitten by the same problem--my XHR was using POST--and only after stumbling upon this GitHub issue did I learn the default was GET. (Better, perhaps, would be that if you don't specify, it would match either).