Create a `UPGRADING.md` document

[julienrbrt]

Summary

In addition to the migration documentation, we should inspire ourselves from tendermint's UPGRADING.md and have one in this repo.

Problem Definition

What problems may be addressed by introducing this feature?

There is no clear documentation on how to upgrade version and use the new features apart from the migrations guide.

What benefits does the SDK stand to gain by including this feature?

The UPGRADING.md could be seen as a reference document for understanding the added changes and new features of a particular version of the SDK, without having to fully read the details of the migration guide.

For complete exhaustiveness of all the changes, the reference should still be the changelog, and for a brief overview the releases notes should be read.

Are there any disadvantages of including this feature?

Someone needs to go through the CHANGELOG.md before each release and update the upgrading guide, in additions to the migrations guide.

The disadvantage is negligible compared to the benefit added to developers.

Proposal

• Create an UPGRADING.md in the repository
• Let someone (a devrel?) each release make a guide from the CHANGELOG.md of the changes
• Append the guide to the UPGRADING.md

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged
• Contributor assigned/self-assigned

[alexanderbez]

I think such an idea can only help, so I'm all for it! 🙌