screen: add README (#8256)

plugins/screen/README.md

+# screen
+
+This plugin sets title and hardstatus of the tab window for [screen](https://www.gnu.org/software/screen/),
+the terminal multiplexer.
+
+To use it add `screen` to the plugins array in your zshrc file.
+
+```zsh
+plugins=(... screen)
+```