[ZIP 1] Network Upgrade Policy and Scheduling

[daira]

Next steps in the ZIP process:

• Finish draft in [ZIP 1]: Network Upgrade Process and Scheduling #1075.

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Document the upgrade schedule and related policy, including how consensus change proposals (from Zcash company or otherwise) are triaged for inclusion.

See also https://github.com/zcash/zcash/issues/2308 .