Add example on how to install it

[benkeil]

I'm bloody new to packer and it's not obvious how to install this plugin.

packer plugins install 'github.com/mkaczanowski/packer-post-processor-flasher' master                                                                                                        
Invalid source string "github.com/mkaczanowski/packer-post-processor-flasher": Plugin source has a type with the "packer-" prefix, which isn't valid.
If you are the author of this plugin, rename it to not include the prefix.
Ex: "post-processor-flasher"

What is the correct way to install it?