doc improve

docs/new-extensibility-model/extension-guides/command/command.md

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 ### Adding custom images
 
 To add custom images which you can later reference with custom monikers, you will need the following steps:
-1. Rename the image source files to `%Custom Moniker%.*`, files prefixed with the same moniker will all be used as backing sources for the same image. Different source will be used based on the requested icon size.
+1. Rename the image source files to follow the `%Custom Moniker%.*` pattern (e.g: `MyMoniker.1.png`). Files prefixed with the same moniker will all be used as backing sources for the same custom moniker. Different source will be used based on the requested icon size.
 	- For example, `MyImage.16.16.png` (a 16\*16 png), `MyImage.20.20.png` (a 20\*20 png) and `MyImage.xaml` are all considered as sources for `MyImage`. 
 	- When the requested icon size is 16*16, `MyImage.16.16.png` will be used, when requested size is 20\*20, `MyImage.20.20.png` will be used, in all other cases, `MyImage.xaml` will be used. 
 2. Put all of the image source files under `Images` folder.