Place the main.js in lib folder and performcheck.js in the data folder and that should be fine. Make sure "main": "main" is set in the package.json file (no need to mention this line if it is missing default is main.js)
There is no need to modify install.rdf its job is for describing the addon and compatibility.
Further information can be read from this documentation link https://developer.mozilla.org/en-US/Add-ons/SDK/Tutorials