Added: readme.txt for Snap

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branch : develop

dist/snap/readme.txt

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+- to install snapcraft : sudo apt-get install snapcraft snapd
+- to generate snap : snapcraft
+- to clean build directory : snapcraft clean ryzom --step build
+- to install a .snap : sudo snap install <file .snap> --dangerous
+- to uninstall an installed .snap : sudo snap remove ryzom