Added bookmark manager tutorial

[segphault]

• [ ] Check blog post for grammatical errors and punctuation
• [ ] Have another person review the post for veracity / readability
• [ ] All links are given in reference style and double checked for correct hrefs
• [x] The text is hard-wrapped
• [ ] RethinkDB relevant content should be links to the docs / api versus reproduced in the post itself, eliminating duplicate work later.
• [ ] A @RethinkDB tweet is formulated and ready to go for this post and included in the pull request.
• [ ] Work with @ahruygt to find the right hero image
• [ ] Images should be put under <www-thoughtbot dir>/assets/posts/images/<title-of-post>-stub.jpg|png
• [ ] Ensure that <!--more--> exists for the post excerpt
• [ ] Ensure that all headers are set to <h1> via the # MD syntax
• [ ] Having lists of links only highlight the verb and object
• [ ] Ensure that the date in the filename is consistent with publication date
• [ ] Ensure that any code examples in GitHub repos or Gists are linked appropriately

[dalanmiller]

Please wait 60 seconds for container to build the branch. https://deploy.rethinkdb.com:32772