Add deprecation guide for ds.deprecated-initializers

[jenweber]

• RFC: Add RFC for deprecation Ember Data initializers rfcs#181
• Ember PR: Deprecate unused initializers warp-drive-data/warp-drive#4657

id: 'ds.deprecated-initializers',
until: '3.0.0'

@locks should this go under a pending features header or elsewhere? Is there anything else I should know besides what is in the RFC? This seems pretty straightforward.

[locks]

@locks should this go under a pending features header or elsewhere?

Yup, pending features sounds good!

Is there anything else I should know besides what is in the RFC?

Nope, that's it really, very straightforward update of a string!