safe-paste: add README (#8292)

plugins/safe-paste/README.md

+# safe-paste
+
+Preventing any code from actually running while pasting, so you have a chance to review what was actually pasted before running it.
+
+To use it, add `safe-paste` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... safe-paste)
+```