Adds EntityEmbedPostRenderCache service class.
Added docs for core patch.
Merge branch '8.x-1.x' into 2247779-post-render-callback-service

Conflicts:
	entity_embed.services.yml
Merge branch '8.x-1.x' into 2247779-post-render-callback-service

Conflicts:
	README.md
Added EntityHelperTrait::renderEntityEmbedDisplayPlugin().
Merge pull request #38 from drupal-media/2247779-post-render-callback-service

Added an EntityEmbedPostRenderCache service class in response to https://drupal.org/project/2247779 committed to core.