[encrypted-media] Provide best practices for authors

ddorwin has just created a new issue for 
https://github.com/w3c/encrypted-media:

== Provide best practices for authors ==
Similar to 
https://html.spec.whatwg.org/multipage/embedded-content.html#best-practices-for-authors-using-media-elements,
 provide best practices for authors, especially based on learings and 
applicaiton implementation experience. For example, authors may want 
to call `close()` and wait for the promise if they are concerned about
 resource usage.

Please view or discuss this issue at 
https://github.com/w3c/encrypted-media/issues/360 using your GitHub 
account