Add deprecation checklist document for Substrate (#1583)

fixes https://github.com/paritytech/polkadot-sdk/issues/182

This PR adds a document with recommendations of how deprecations should
be handled. Initiated within FRAME, this checklist could be extended to
the rest of the repo.

I want to quote here a comment from @Kianenigma

 that summarizes the
spirit of this new document:
> I would see it as a guideline of "what an extensive deprecation
process looks like". As the author of a PR, you should match this
against your "common sense" and see if it is needed or not. Someone else
can nudge you to "hey, this is an important PR, you should go through
the deprecation process".
> 
> For some trivial things, all the steps might be an overkill.

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Co-authored-by: Francisco Aguirre <[email protected]>