Add documentation about how to use .js attachements

[franckaubry]

Please expand the documentation regarding attachements to also explain how to use classes and functions in Cables.

The syntax for retrieving strings from attachements is very well explained. Something similar explaining calling functions and also the "inc_xxx.js" naming convention would be good to know about when developing ops and splitting it up into multiple attachements.
I suspect that "inc_" files are loaded before the main .js file for instance?

[pandrr]

yes they are executed before the main op code.