Examine source code of rym2lfm

{
  "manifest_version": 2,
  "name": "rym2lfm",
  "version": "0.2",
  "description": "Creates a button on a rateyourmusic.com release page that links to the corresponding Last.fm page.",
  "icons": {
    "48": "icons/48.png",
    "64": "icons/64.png"
  },
  "content_scripts": [
    {
      "matches": [
        "*://*.rateyourmusic.com/release/*"
      ],
      "js": [
        "script.js"
      ]
    }
  ]
}