Add documentation for codec column configuration

[PJDuszynski]

Summary

Adds a new section in the README containing information about column-level configuration options.

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• [ x] A human-readable description of the changes was provided to include in CHANGELOG

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[arun11299]

@PJDuszynski Can you provide an example of column configuration as well ? I am putting it under schema.yml. Someghting like below:

models:
  - name: dim_employee
    description: "A starter dbt model"
    columns:
      - name: updated_at
        description: "Update time"
        data_tests:
          - not_null
        codec: "DoubleDelta"

Also tried:

models:
  - name: dim_employee
    description: "A starter dbt model"
    columns:
      - name: updated_at
        description: "Update time"
        data_tests:
          - not_null
        meta:
          codec: "DoubleDelta"

But this does not seem to work. I do not see the codec being applied in the DDL.