Add note about the Apple Developer Program fee

[luhring]

trafficstars

I was trying out the steps listed to get a Developer ID certificate, and I realized I needed to:

1. enroll in the Apple Developer Program
2. pay $99 for the enrollment

(Without completing these steps, the "certificates" link in the README.md doesn't work.)

I thought it'd be good to give users a heads up about these required steps in case they don't already know this, particular for folks coming from the Go development world more so than the Apple development world.

[gedw99]

It really pisses me off how they charge for this. Anyways.

[luhring]

Cleaning out my open PRs, this one's pretty old. Feel free to use if it's ever helpful :)