Documentation enhancements

[GreenishFlow]

Thanks for this plugin.
I found few things didn't mentioned in the documentation.

1. The order is important inside metadata.choice (at least for the choosing resolution).
   So it is matter to have it in a right order. From less resolution to highest.
   Correct:
   metadata: {
   choice: ["@1x.json", "@2x.json"]
   }
   Incorrect:
   metadata: {
   choice: ["@2x.json", "@1x.json"]
   }
   in a second way it will load 1x for resolution 2.
2. If you are using detectExtensions method, you have to use suffix "@1x" ("@2x" etc), you cannot use "@x1", "@x2" etc.