Add leonardo folder for leonardo + usb2.0 shield based projects

Leonardo is similar to the hasu board but uses caterina as bootloader.

keyboards/converter/usb_usb/leonardo/info.json

@@ -0,0 +1,3 @@
+{
+    "bootloader": "caterina"
+}

keyboards/converter/usb_usb/leonardo/rules.mk

@@ -0,0 +1,2 @@
+# Processor frequency
+F_CPU = 16000000

keyboards/converter/usb_usb/readme.md

@@ -11,6 +11,10 @@ Make example for this keyboard (after setting up your build environment):
 
     make converter/usb_usb:default
 
+If you use Arduino Leonardo with a USB Host Shield:
+
+    make converter/usb_usb/leonardo:default
+
 See [build environment setup](https://docs.qmk.fm/#/getting_started_build_tools) then the [make instructions](https://docs.qmk.fm/#/getting_started_make_guide) for more information.
 
 Note that you have to choose the right hardware variant as your subproject, otherwise you will probably have issues.
@@ -23,6 +27,7 @@ If you are sure you have this correct, try changeing the default in `usb_usb/rul
 
 The Pro Micro variant uses a 3.3V Pro Micro and thus runs at 8MHz, hence the following line in `usb_usb/pro_micro/rules.mk`:
 `F_CPU = 8000000`
+
 The converter sold by Hasu runs at 16MHz and so the corresponding line in `usb_usb/hasu/rules.mk` is:
 `F_CPU = 16000000`