Updates readme file and converts it to markdown.

This is a squash of the following commits on sudhansu/readme-changes:
* a048db3 Updated Readme file. Changed sentence structure for better
  readability and updated the content to match latest Drupal readme
  standards
* f9043ba Updated Readme file. Changed sentence structure for better
  readability and updated the content to match latest Drupal readme
  standards
* 561e772 Updated Readme.md according to
  https://github.com/marvil07/vote_up_down/pull/1/files/f9043ba7c910dee3f17c43da4f3d2603d55ee554
* ad5ed68 Fixed the markdown syntax, used www d.o. URL and rectified
  structual problems
* 93c8ea2 Fixed d.o. URLs