doc(README): added `HelperCoders` usage

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (4eb999c) 95.06% compared to head (4748c79) 95.06%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #38 +/- ## ======================================= Coverage 95.06% 95.06% ======================================= Files 76 76 Lines 1823 1823 ======================================= Hits 1733 1733 Misses 90 90 ``` | [Files](https://app.codecov.io/gh/SwiftyLab/MetaCodable/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SwiftyLab) | Coverage Δ | | |---|---|---| | [Sources/HelperCoders/PropertyWrapperCoder.swift](https://app.codecov.io/gh/SwiftyLab/MetaCodable/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SwiftyLab#diff-U291cmNlcy9IZWxwZXJDb2RlcnMvUHJvcGVydHlXcmFwcGVyQ29kZXIuc3dpZnQ=) | `100.00% <ø> (ø)` | | | [Sources/MetaCodable/HelperCoders/HelperCoder.swift](https://app.codecov.io/gh/SwiftyLab/MetaCodable/pull/38?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=SwiftyLab#diff-U291cmNlcy9NZXRhQ29kYWJsZS9IZWxwZXJDb2RlcnMvSGVscGVyQ29kZXIuc3dpZnQ=) | `75.00% <ø> (ø)` | |

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