Add information about dealing with deprecations generally

[locks]

Since we have an index page for deprecations, we should strive to utilize it better.
One thing we could add is generic information on how to deal with deprecations, and the various tools available.

These are more suggestions than requirements:

• Mention https://github.com/mixonic/ember-cli-deprecation-workflow
• Mention https://github.com/kellyselden/ember-cli-update