docs: adds readme to test-helper package

packages/test-helper/README.md

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+# APIMatic Test Helper Libary for JavaScript
+
+> This library is currently in preview.
+
+Provides assertion utilities for testing api calls. It can be plugged in as dev dependency to any library.
+
+The exported helper functions include:
+
+1. **getStreamData**: Get streaming data from a given URL.
+2. **areStreamsMatching**: Check if input matches the contents of a file by comparing stream data or Blob data.
+3. **expectHeadersToMatch**: Compare actual headers with expected headers, ignoring case sensitivity.
+4. **isProperSubsetOf**: Recursively check whether the left object or array is a proper subset of the right object or array.
+5. **isOrderedSupersetOf**: Check if one ordered array is a superset of another ordered array.
+6. **isSuperSetOf**: Check if one array is a superset of another array.
+
+This library is used as a dev-dependency by JavaScript SDKs generated by the [APIMatic Code Generator](http://www.apimatic.io).
+
+## Builds
+
+The following environments are supported:
+
+1. Node.js v10+
+1. Bundlers like Rollup or Webpack
+1. Web browsers
+
+To support multiple environments, we export various builds:
+
+| Environment | Usage |
+| --- | --- |
+| Common.js | Import like this: `require('@apimatic/test-helper')`. |
+| ES Module | Import like this: `import { /* your imports */ } from '@apimatic/test-helper'`. |
+| Browsers | *Use script: `https://unpkg.com/@apimatic/test-helper@VERSION/umd/schema.js` |
+| Modern Browsers (supports ESM and uses modern JS) | *Use script: `https://unpkg.com/@apimatic/test-helper@VERSION/umd/schema.esm.js` |
+
+_* Don't forget to replace VERSION with the version number._
+
+**Note**: We discourage importing files or modules directly from the package. These are likely to change in the future and should not be considered stable.