Add BackstopJS installation instructions

[cleverington]

Installation (global): npm install -g backstopjs
Installation (local): npm install --save-dev backstopjs

Configuration: backstop init (Creates backston.json template)

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Commands

• backstop reference - Create a series of baseline/reference images for future tests to be conducted against
• backstop test - Take a new series of screenshots and compare them to the reference set, then open a report
• backstop accept - Use the last set of screenshots as the new reference, accepting them as the new canonical version to test against
• backstop approve - Promotes all test bitmaps from last test run to reference bitmaps
• backstop openReport - View the last test report in your browser

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Extras

• Backstop Crawl
• Backstop Generator - https://www.drupal.org/project/backstop_generator/