docs(jsontools): document requirements

Fixes #10834

plugins/jsontools/README.md

@@ -8,6 +8,17 @@ To use it, add `jsontools` to the plugins array in your zshrc file:
 plugins=(... jsontools)
 ```
 
+## Requirements
+
+The plugin uses one of these tools to process JSON data, in the following order:
+
+- `node`
+- `python3`
+- `ruby`
+
+Any of these must be in `$PATH` before the plugin is loaded, otherwise the plugin exits
+prematurely and the functions will not be available.
+
 ## Usage
 
 Usage is simple... just take your json data and pipe it into the appropriate jsontool: