Add notice when pretty permalinks are not being used

[salcode]

See #32

[joehoyle]

We should just fix working without permalinks

[salcode]

@joehoyle That is a great idea but I'd still like to see this warning put in place as a temporary fix (until it works without pretty permalinks)

[salcode]

Now that WP 4.7 is out with the REST API in it - congratulations! Along with this, I'm guessing this plugin will increase in popularity.

On Sunday I was at WCUS's contributor day. When I decided to contribute to core I started out with this plugin on a fresh install to test endpoints and I got this cryptic error. I then posted a message to slack and got no response. I finally asked one of the organizers regarding testing the endpoints about this error and the suggestion I got was to use a different tool to test the endpoints. This was a lousy user experience, which is bad for the REST API reputation and WordPress's reputation.

Adding this notice will prevent others from having a similar experience.

I won't badger any further regarding this PR but I wanted to share my motivation for it.

Cheers.