Update README.md

README.md

@@ -315,6 +315,20 @@ git clone https://github.com/john/my-project
 > [!NOTE]
 > After you read a variable such as `project_name`, in any file you update or copy, `{{ tmplr.project_name }}` will be replaced with the value read. If a variable is not resolved, then `tmplr` will leave it untouched.
 
+> [!IMPORTANT]
+> Make sure any template variable you use starts with `tmplr.` prefix. tmplr will ignore any variable that doesn't.
+> 
+> ```yaml
+> # ❌ this is wrong, and `{{ project_name }}` will remain untouched
+> project:
+>   name: {{ project_name }}
+> ```
+> ```yaml
+> # ✅ this is correct. `{{ tmplr.project_name }}` will be replaced by the value read by the recipe.
+> project:
+>  name: {{ tmplr.project_name }}
+>```
+
 <br>
 
 We have multiple _commands_ and _expressions_ used in the example recipe above: