message: Add migration for v0.20.0 from v0.19.0
  It was reported that when upgrading from v0.19 to v0.20 an error about requiring new was exposed. Adding a note about this change to the changelog
diff: @@ -163,6 +163,21 @@ const pubsub = new PubSub();
  *BREAKING CHANGE*: - fix: drop support for node.js 4.x and 9.x (#171)
  +
  +**BREAKING CHANGE**
  +`@google-cloud/pubsub` now requires `new`.
  +
  +Before:
  +```javascript
  +const PubSub = require('@google-cloud/pubsub');
  +const pubsub = PubSub();
  +```
  +Now:
  +```javascript
  +const PubSub = require('@google-cloud/pubsub');
  +const pubsub = new PubSub();
  +```
  +
  ### New Features
  - Re-generate library using /synth.py (#227)
  