terminitor: add README (#8197)

plugins/terminitor/README.md

+# Terminitor plugin
+
+This plugin adds completions for the [Terminitor](https://github.com/achiurizo/terminitor) development workflow setup tool.
+
+To use it, add `terminitor` to the plugins array in your zshrc file:
+
+```zsh
+plugins=(... terminitor)
+```