CONTRIBUTING: be more explicit about the "help wanted" label
CONTRIBUTING: be more explicit about the "help wanted" label

Also add `docs/api.md` to the list of documentation files.
electron-userland/electron-packager,Goobaroo/electron-packager,sethlu/electron-packager,malept/electron-packager,malept/electron-packager,Goobaroo/electron-packager,maxogden/electron-packager,sethlu/electron-packager,electron-userland/electron-packager