battery: add README (#7309)

plugins/battery/README.md

+# Battery Plugin
+
+This plugin adds some functions you can use to display battery information in your custom theme.
+
+To use, add `battery` to the list of plugins in your `.zshrc` file:
+
+`plugins=(... battery)`
+
+Then, add the `battery_pct_prompt` function to your custom theme. For example:
+
+```
+RPROMPT='$(battery_pct_prompt)'
+```