Added definition, example, methods, resources

[jeremybbauer]

Added the definition (similar to the "qualitative surveys" article), example, list of testing methods, the source of the definition, and resources.

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• Added descriptive name to PR
  • Your PR should NOT be called Update index.md since it does not help the maintainer understand what has been changed.
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  • Other examples are Git: edit Git Commit article or PHP: create PHP section and add Data Structures article
• Reviewed necessary formatting guidelines in CONTRIBUTING.md.
• No plagiarized, duplicate or repetitive content that has been directly copied from another source.

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