Added migration notes for moving to vonage packages

[dragonmantank]

Description

This adds a notice that this repo is officially in maintenance mode and points users to https://github.com/vonage/vonage-php-sdk-core. It also adds migration instructions to help users upgrade.

Motivation and Context

As we move forward with the rebrand, we need to help customers update. This provides notes and instructions. Looking for copy editing and notes on this for clarity.

How Has This Been Tested?

It's a textual change

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-commenter]

Codecov Report

Merging #2 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master       #2   +/-   ##
=========================================
  Coverage     80.31%   80.31%           
  Complexity     2396     2396           
=========================================
  Files           166      166           
  Lines          6072     6072           
=========================================
  Hits           4877     4877           
  Misses         1195     1195           

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[slorello89]

LGTM