Define migration steps for users upgrading to the rebased version

[ch4ot1c]

Documentation is needed for users who will be upgrading from BTCP 1.0.12-1. Items include:

• [x] Discuss wallet.dat compatibility issues + solutions, if any.
• [ ] Discuss other files that would need to be moved from ~/.btcprivate to ~/.btcp.
• [ ] Mention files, env vars that won't need to move/change (e.g. ~/.zcash-params).
• [ ] Present a link to necessary migration scripts to run, if any.
• [x] Present a link to a 'clean start' option.

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[ch4ot1c]

ZMQ port change!

[ch4ot1c]

BDB 4.8 to BDB 6.2 breaks wallet compatibility, see Bitcoin issue 3686

[ch4ot1c]

The ./fetch-zcash-params call should be added into an initial setup script.

[ch4ot1c]

'Clean start' simply needs to be an uninstaller for the prior versions.