Add repo archiving message to README

[ScriptAutomate]

Preparing repository for archiving.

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Project is Archived

• Salt Project / VMware has ended active development of this project, this repository will no longer be updated.
• This project was for classic packaging of Salt back when packages were developed for Python 2.
  • For Python 3, see salt-pack-py3, which will be in use until Salt v3005.x is EOL.
  • Salt v3006, and later, are packaged as onedir releases that take advantage of relative-environment-for-python (relenv). Those packaging builds no longer make use of salt-pack-* toolsets.