Add new documentation for experimental features (#360)

Author: KS1019

Sources/ArgumentParser/Documentation.docc/Articles/ExperimentalFeatures.md

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+# Experimental Features
+Learn about what experimental features ArgumentParser has.
+
+## Overview
+ArgumentParser has some built-in experimental features and they are available with the prefix `--experimental`. If you have any feedback on the experimental features, please open an issue.
+
+## List of experimental features
+### Experimental Flags
+| Name | Description | related PRs | Version |
+| ------------- | ------------- | ------------- | ------------- |
+| `--experimental-dump-help`  | Dumps command/argument/help information as JSON | [#310](https://github.com/apple/swift-argument-parser/pull/310) [#335](https://github.com/apple/swift-argument-parser/pull/335) | 0.5.0 or newer |