Add deprecation notice for old config style

[LandonTClipp]

This fixes #646. The old config style is being officially deprecated in favor of packages. Add documentation that explains this.

[codecov]

Codecov Report

Patch coverage: 100.00000% and project coverage change: -1.27061 ⚠️

Comparison is base (7a91185) 76.54039% compared to head (0040d3d) 75.26978%.

❗ Current head 0040d3d differs from pull request most recent head a43101b. Consider uploading reports for the commit a43101b to get more accurate results

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master        #668         +/-   ##
===================================================
- Coverage   76.54039%   75.26978%   -1.27062%     
===================================================
  Files              8           9          +1     
  Lines           2191        2224         +33     
===================================================
- Hits            1677        1674          -3     
- Misses           378         412         +34     
- Partials         136         138          +2     

Impacted Files	Coverage Δ
cmd/mockery.go	58.95062% <100.00000%> (-3.21154%)	⬇️
pkg/config/config.go	70.56452% <100.00000%> (ø)
pkg/logging/logging.go	30.95238% <100.00000%> (ø)

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