Add doc on zone wrappers

Author: jamesdaniels

docs/zones.md

@@ -0,0 +1,21 @@
+<small>
+<a href="https://github.com/angular/angularfire">AngularFire</a> &#10097; <a href="../README.md#developer-guide">Developer Guide</a> &#10097; Zone Wrappers
+</small>
+
+# Zone Wrappers
+
+AngularFire wraps the [framework agnostic Firebase JS SDK](https://github.com/firebase/firebase-js-sdk) and [RxFire](https://github.com/firebaseextended/rxfire) to ensure proper functionality in Zone and Zoneless applications alike.
+
+These wrappers ensure Firebase APIs are called outside of the Angular zone but return in the Angular zone. This isolates side-effects such as timers so that they do not destabilize your application. 
+
+Observables, Promise-based APIs, and callbacks will await an initial emission before stabilizing your application allowing for proper SSR/SSG functionality
+
+## Consequences of not Zone wrapping
+
+When using a Firebase or RxFire API without importing from AngularFire or if AngularFire APIs are used outside of an injection context you _may_ experience instability.
+
+To help debugging, AngularFire emits warnings when it is unable to Zone wrap an API while in dev-mode. **Often these messages can be safely ignored** but instability can be difficult to track down ([see Resolving zone pollution in the Angular docs](https://angular.dev/best-practices/zone-pollution)), so we'd rather be verbose.
+
+When an application is unstable change-detection, two-way binding, and rehydration may not work as expected—leading to subtle and non-subtle bugs and performance issues in your application. Further, server-side rendering (SSR) and static site generation (SSG/pre-rendering) may timeout or render a blank page.
+
+There are a number of situations where AngularFire's Zone wrapping is inconsequential such adding/deleting/updating a document in response to user-input, signing a user in, etc. So long as no long-lived side-effects are kicked off, your application should be ok. Most Promise based APIs are fairly safe without zone wrapping.