battery: add acpi requirement to README (#9116)

Co-authored-by: Marc Cornellà <marc.cornella@live.com>

plugins/battery/README.md

@@ -9,5 +9,14 @@ To use, add `battery` to the list of plugins in your `.zshrc` file:
 Then, add the `battery_pct_prompt` function to your custom theme. For example:
 
 ```
-RPROMPT='$(battery_pct_prompt)'
+RPROMPT='$(battery_pct_prompt) ...'
+```
+
+## Requirements
+
+On Linux, you must have the `acpi` tool installed on your operating system.
+
+Here's an example of how to install with apt:
+```
+sudo apt-get install acpi
 ```