Rename keyboard-level readmes to lower-case (#10759)

This is the recommendation in the [PR
Checklist](https://docs.qmk.fm/#/pr_checklist?id=keyboard-prs) and it
will make working with them easier in the CLI/API/etc.

keyboards/converter/usb_usb/ble/readme.md

@@ -0,0 +1,30 @@
+QMK BLE Adapter
+===============
+A small device that reads USB keyboard input and passes it on over Bluetooth LE HID.
+
+![Picture of Adapter](https://i.imgur.com/O3VwBBJ.png)
+
+Hardware
+--------
+The hardware is relatively easy to assemble. Just follow the schematic and don't forget to [cut the VBUS jumper](https://www.pjrc.com/teensy/td_libs_USBHostShield_4a.jpg).
+
+Schematic:
+![Schematic](https://i.imgur.com/s1Ia9We.png)
+
+Part list:
+* [Adafruit Feather 32u4 Bluefruit LE](https://www.adafruit.com/product/2829)
+* [USB Host Mini](https://www.circuitsathome.com/arduino_usb_host_shield_projects/)
+* [Pololu 5V Step-Up Voltage Regulator U3V12F5](https://www.pololu.com/product/2115)
+* [Lithium Ion Battery - 3.7v 2000mAh](https://www.adafruit.com/product/2011)
+* Some sort of switch to be able to turn it off
+
+Building and Flashing
+---------------------
+
+```
+make converter-usb_usb-ble
+```
+
+```
+make converter-usb_usb-ble-avrdude
+```