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/handwired/d48/readme.md

@@ -0,0 +1,87 @@
+# D48
+
+![Proton C based handwired 40% keyboard](https://i.imgur.com/2wCYuno.jpg)
+
+A Proton C based handwired 48 key keyboard with 2 rotary encoders, I2C OLED, WS2812 strip, buzzer & clock!
+
+- Keyboard Maintainer: Andrew Dunai
+- Hardware Supported: Proton C handwired
+
+Make example for this keyboard (after setting up your build environment):
+
+    make handwired/d48:default
+
+## Details
+
+- Proton C based handwired keyboard
+- 2x custom 1.25mm stainless steel plates
+- Kailh Choc White (clicky)
+- 2x rotary encoders
+- 0.91" 128x32 I<sup>2</sup>C OLED
+- Small buzzer mounted inside (still waiting for the AST1109MLTRQ boys)
+- WS2812 strip (14 LEDs)
+- DS1307 I<sup>2</sup>C module real-time clock
+
+Build process: [album](https://imgur.com/gallery/zZZGdDw)
+
+## Pinout
+
+![D48 pinout](https://imgur.com/QoStCvD.jpg)
+
+## Challenges
+
+I'm very happy with the result, but at some point Proton C was driving me nuts.
+
+I did a lot of trial and error during assembly & programming.
+There were a lot of *yet* undocumented caveats,
+so I'll outline them here so that you guys can avoid the same issues I had.
+
+## Matrix & encoders
+
+Although this is a 48-key board with a 12-col & 4-row matrix, I've decided to add an extra row above the first one
+to make my matrix 12x5 and wire encoders' push buttons as 2 extra keys, thus making it a total of 50 (12x4 + 2 encoders).
+I used 2 columns (9 & 12) for those buttons.
+
+So, a first row actually has 2 buttons on columns 9 & 12 (because encoders are located near those columns).
+Encoders' push buttons are also configured via QMK's keymap.
+
+Check out the `d48.h` & `config.h` for pins used & keymap macro definition.
+
+## I<sup>2</sup>C/OLED
+
+Most of the stuff worked out of the box, except me choosing the right pins for my OLED.
+
+On the Proton C pinout, there are 3 labels for I<sup>2</sup>C and for some reason
+there are 2 pairs of SDA/SCL for I<sup>2</sup>C<sup>1</sup> channel: `B8`/`B9` (rear left side) and `B6`/`B7` (rear right side).
+I'm not sure if this is a mistake or if I was doing something wrong. So initially I picked `B8`/`B9`
+which were not working. When I switched to B6/B7, things worked like a charm.
+Later I used B9 for matrix row. No issues so far.
+
+Oh, and by the way, while using `B8`/`B9`, keyboard was sometimes *swallowing* quick keypresses.
+I believe this was due to I(2)C timeouts (because incorrect pins were used for OLED).
+
+## Buzzer
+
+It turns out once you switch on `AUDIO_ENABLE`, you cannot use A4 & A5 because they interfere with the buzzer.
+My guess is that buzzer uses DAC channels (not sure why both).
+
+I couldn't find this in documentation. Honestly, Proton C has almost zero documentation and this was
+the biggest challenge. Anyway, apart from almost going crazy from those challenges, I really liked it!
+
+## RGB
+
+I used pin `A15` for my WS28128 RGB strip.
+
+## D1307 real-time clock
+
+Connecting DS1307 RTC was a piece of cake: same I<sup>2</sup> pins as OLED (SDA/SCL), GND to GND and power to Proton C VUSB pin (5v).
+
+## Other issues
+
+- `B5` could not be used for matrix.
+- `TAP_CODE_DELAY` had to be increased to 10 to fix `tap_code(KC_VOLU/KC_VOLD)` calls being swallowed in encoder callback.
+- Be extremely attentive about the pinout: keep in mind that **the official Proton C pinout displays the rear of the board, not the front.** Being used to front pinouts, I ended up soldering entire matrix to the wrong side, so I had to desolder every wire and connect it to the opposite side.
+
+## Conclusion
+
+I had a lot of fun. The layout was inspired by the Planck THK. Feel free to ask any questions!