Added how to build a plugin into the readme

Signed-off-by: shennetsind <ind@henn.et>
Added how to install a plugin into the main makefile

Signed-off-by: shennetsind <ind@henn.et>
Added how to build a plugin into Yommy/Readme.md

Signed-off-by: shennetsind <ind@henn.et>
Updated SkillErrorMessages/README.md

forgot this one

Signed-off-by: shennetsind <ind@henn.et>