Add suggestion to Changelog.md for removal of compose

[joshalling]

compose gets removed in 3.0.0. This suggestion would have been helpful to me when I upgraded.

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes)
• Make sure all of the significant new logic is covered by tests
• If this was a change that affects the external API used in GitHunt-React, update GitHunt-React and post a link to the PR in the discussion.

[apollo-cla]

@joshalling: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Meteor Contributor Agreement here: https://contribute.meteor.com/

[hwillson]

Thanks for this @joshalling!