Add contribute.md about readium

[sudodoki]

@starburst1977 could you explain why you closed this issue? Didn't see any recent commits that would fix this.

[bkroggel]

@sudodoki looks like he pulled up a entirely new made theme of Readium and closed all issues related to the former version.

[starburst1977]

@sudodoki Yes, I remade the whole theme. So there is nothing to compile anymore (besides the SASS files for CSS). So I closed all issues the where entirely relate to the old version of the theme. Let me know if you still have problems.

[sudodoki]

besides the SASS files for CSS

@starburst1977 would still appreciate one-liner that will output the result just as in current theme output.
Took me some time (because haven't worked with LESS for some time) to figure out
lessc theme_file.less -x > theme_file.css to work with previous theme

[starburst1977]

@sudodoki I would recommend using Codekit for compiling. That's what I use, because I am no command-line-hero.