pull retropad out of handwired and update readme.md (#11545)

Co-authored-by: Swiftrax <swiftrax@gmail.com>

keyboards/swiftrax/retropad/retropad.h

@@ -0,0 +1,31 @@
+/* Copyright 2020 swiftrax
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 2 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>.
+ */
+#pragma once
+
+#include "quantum.h"
+
+// readability
+#define XXX KC_NO
+
+#define LAYOUT( \
+       K001,    \
+    K100, K101, \
+    K200, K201  \
+) { \
+    {  XXX,  K001, }, \
+    { K100,  K101, }, \
+    { K200,  K201, }  \
+}