Document injection into head element

[brodybits]

Document that Capacitor assumes it can inject itself into the head element

as discussed in:

• Unable to inject Capacitor, Plugins won't work #1572 (comment)
• Unable to inject Capacitor, Plugins won't work #1572 (comment)
• https://forum.ionicframework.com/t/unable-to-inject-capacitor/178713

I ran into this when migrating a small test app from Cordova to Capacitor, and it took me some research to find the solution.

I will completely understand if this proposal needs to be cleaned up, just hoping we can help someone else avoid this kind of a pitfall.

[imhoffd]

@brodybits Apologies this didn't make it in. We split out the Capacitor site to its own repo: https://github.com/ionic-team/capacitor-site

Would you mind recreating this there?