Update Coffeescript/Javascript development readme

[Henni]

Currently the readme for the coffeescript development is describing a compilation with 'make'.
But as there is a gruntfile there should be an explanation on which grunt commands to execute to get the compiled and concatenated public/app.js file.

Maybe the gruntfile has to be redone as well, but I'm not sure about that.

[raimund-schluessler]

@Henni Do you consider this done with the changes merged with c733b02 ?

[Henni]

Yes I think this is done