pass: add README (#8282)

plugins/pass/README.md

+# pass
+
+This plugin provides completion for the [pass](https://www.passwordstore.org/) password manager.
+
+To use it, add `pass` to the plugins array in your zshrc file.
+
+```
+plugins=(... pass)
+```
+
+## Configuration
+
+### Multiple repositories
+
+If you use multiple repositories, you can configure completion like this:
+```zsh
+compdef _pass workpass
+zstyle ':completion::complete:workpass::' prefix "$HOME/work/pass"
+workpass() {
+  PASSWORD_STORE_DIR=$HOME/work/pass pass $@
+}
+```