Document that testing requires trusted certificates

[iandunn]

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The service workers won't work properly in local environments with self-signed certs, unless those certs are fully trusted by the browser (i.e., green bar). It'll be important to tell devs that when they're working with this, since up until know you can usually get away with simply adding an exception to the browser, rather than going through the extra hassles of generating certs that can be trusted by the major browsers, and then trusting them.

[iandunn]

It'd probably be helpful to point to an good article for setting them up, since most of the ones out there won't result in a green bar, if they even work at all. This is a good one (or at least it worked for me):

https://deliciousbrains.com/ssl-certificate-authority-for-local-https-development/

[iandunn]

mkcert looks like it might also be a good resource to link to.