Updated README.txt to README.md
requested to merge issue/extrafield_views_integration-3319925:3319925-replace-readme.txt-with into 2.x
Updated README.txt to README.md.
Also reviewed the patch(3319925-6.patch) by komalparmar, The patch didn't apply cleanly, Had errors.
Also updated the Markdown render for code block as per the Drupal Markdown practices and addressed the pointers by abramm #4.
Please review the same.