Add migration guide

[emlaver]

Checklist

• [x] Tick to sign-off your agreement to the Developer Certificate of Origin (DCO) 1.1
• [x] Added tests for code changes or test/build only changes
• [x] Updated the change log file (CHANGES.md) or test/build only changes
• [x] Completed the PR template below:

Description

Add migration guide from python-cloudant to new beta cloudant-python-sdk library. Guide includes a reference mapping table that links python-cloudant to cloudant-python-sdk API operation documentation link.

Approach

Schema & API Changes

Security and Privacy

Testing

Monitoring and Logging

[emlaver]

@mojito317 @vmatyusGitHub thanks for the review! I've addressed your comments in 0a18897.