docs(guides): add guide for using compatibility mode

Fixes #4018

Author: epelc

guides/compatibility-mode.md

@@ -0,0 +1,8 @@
+## What is compatibility mode?
+Compatibility mode for `@angular/material` allows the components to exist side-by-side with AngularJS Material components without any CSS collisions.
+
+## What does compatibility mode do?
+When enabled, compatibility mode enforces that all template APIs use the prefix `mat-` instead of `md-`. This will prevent any CSS from AngularJS Material from affecting the components in `@angular/material`.
+
+## How is compatibility mode enabled?
+Import `NoConflictStyleCompatibilityMode` into your application's root `NgModule`.