docs: document source directory usage

Change-Id: Id2edadaaae78544736a0ce1156aca3ff9e003097
Signed-off-by: Marc Kupietz <kupietz@ids-mannheim.de>
diff --git a/Readme.md b/Readme.md
index 7cf29b9..e08d652 100644
--- a/Readme.md
+++ b/Readme.md
@@ -4,9 +4,18 @@
 
 ## Usage
 
+By default, place your `.i5.xml` files in the `I5` directory in the root of the project:
+
 ```bash
 make
 ```
+
+Alternatively, you can specify a different source directory containing your `.i5.xml` files using the `SRC_DIR` variable:
+
+```bash
+make SRC_DIR=/path/to/your/files
+```
+
 Then open http://localhost:64543 in your browser.
 
 ## Prerequisites