Clarify licensing terms for the contents of the repository

[aperezdc]

Currently there is no indication about which license covers the different kinds of source content in this repository, which might prevent people from contributing and/or picking content for other purposes like creating documentation for WPE related projects which reuses snippets or pictures from the web site.

This may need some discussion on which particular license to use. I think we can easily agree on some free/libre license for text content, but I expect some that we might want to consider disallowing modifications of the logos, to make sure they remain recognizable, and maybe some other elements.