added missing usb config file, keypad submodule and readme

.gitmodules

@@ -7,3 +7,6 @@
 [submodule "sdcard"]
 	path = sdcard
 	url = https://github.com/grblHAL/Plugin_SD_card
+[submodule "keypad"]
+	path = keypad
+	url = https://github.com/grblHAL/Plugin_I2C_keypad

README.md

@@ -0,0 +1,9 @@
+## RP2040 Driver
+
+A grblHAL driver for the Raspberry Pi RP2040 processor on a [Pi Pico board](https://www.raspberrypi.org/products/raspberry-pi-pico/).
+
+This driver is currently [work in progress](https://github.com/terjeio/grblHAL/discussions/206#discussioncomment-318652) and not ready for use.
+
+
+---
+2021-03-11

tusb_config.h

@@ -0,0 +1,36 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2020 Raspberry Pi (Trading) Ltd.
+ * Copyright (c) 2020 Damien P. George
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ *
+ */
+
+#ifndef _PICO_STDIO_USB_TUSB_CONFIG_H
+#define _PICO_STDIO_USB_TUSB_CONFIG_H
+
+#define CFG_TUSB_RHPORT0_MODE   (OPT_MODE_DEVICE)
+
+#define CFG_TUD_CDC             (1)
+#define CFG_TUD_CDC_RX_BUFSIZE  (256)
+#define CFG_TUD_CDC_TX_BUFSIZE  (256)
+
+#endif