Added schema picture to README_imgs

Created a schema for our database. It contains three tables representing the three models we will need for our hacker news clone:  a post, a comment, and a user.  The comment is a join table between point and user.
Added wireframe.png and schema.png pictures to README.md

Learned how to use relative links on github to put pictures of
the wireframe and schema in the readme. Relative links:
https://help.github.com/articles/relative-links-in-readmes