Clarify readme and contributing

Greg asked volunteers to review these two files ([Discuss] volunteer wanted to clarify README.md and CONTRIBUTING.md) and I did. Feel free to comment.
I don't like the "certain conditions" because looks like there is a form that says if a format is suitable or not.

What about

>  Lessons should be written in Markdown but is possible to use IPython Notebook, RMarkdown or other format. As explained in [the README file](README.md), Jekyll (the tool GitHub uses to create websites) knows how to convert Markdown to HTML (the format that web browser render) and any other file format will be available as it is. For IPython Notebooks, RMarkdown and any other file format the author of the lesson must add the generated Markdown to the repository since this ensures that people who *aren't* familiar with some format don't have to install the tools needed to work with it.