Added "UI Designer" plugin to README prerequisites list
Added "UI Designer" plugin to README prerequisites list

Needed for RustRunSettingsEditor, throws NPE without it.
nathansgreen/idea-rust,nathansgreen/idea-rust,AndrewTamm/idea-rust,AndrewTamm/idea-rust,ZJU-Shaonian-Biancheng-Tuan/idea-rust,tempbottle/idea-rust,ZJU-Shaonian-Biancheng-Tuan/idea-rust,ZJU-Shaonian-Biancheng-Tuan/idea-rust,AndrewTamm/idea-rust,nathansgreen/idea-rust,tempbottle/idea-rust,tempbottle/idea-rust