docs(snap): Remove usage instructions from readme

[farshidtz]

Usage instructions are being moved to docs; see edgexfoundry/edgex-docs#751.
The snap/README has been updated to include development docs only.

Depends on:

• feat: Extend snap getting started docs edgex-docs#751

Signed-off-by: Farshid Tavakolizadeh [email protected]

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PR Checklist

Please check if your PR fulfills the following requirements:

• I am not introducing a breaking change (if you are, flag in conventional commit message with BREAKING CHANGE: describing the break)
• I am not introducing a new dependency (add notes below if you are)
• I have added unit tests for the new feature or bug fix (if not, why?)
• I have fully tested (add details below) this the new feature or bug fix (if not, why?)
• I have opened a PR for the related docs change (if not, why?)

Testing Instructions

New Dependency Instructions (If applicable)