doc updates

docs/proton_c_conversion.md

@@ -0,0 +1,21 @@
+# Converting a board to use the Proton C
+
+If a board currently supported in QMK uses a Pro Micro (or compatible board) and you want to use the Proton C, you can generate the firmware by appending `CONVERT_TO_PROTON_C=yes` (or `CTPC=yes`) to your make argument, like this:
+
+    make 40percentclub/mf68:default CTPC=yes
+
+You can add the same argument to your keymap's `rules.mk`, which will accomplish the same thing.
+
+This exposes the `CONVERT_TO_PROTON_C` flag that you can use in your code with `#ifdef`s, like this:
+
+    #ifdef CONVERT_TO_PROTON_C
+        // Proton C code
+    #else
+        // Pro Micro code
+    #endif
+
+Before being able to compile, you may get some errors about `PORTB/DDRB`, etc not being defined, so you'll need to convert the keyboard's code to use the [GPIO Controls](internals_gpio_control.md) that will work for both ARM and AVR. This shouldn't affect the AVR builds at all.
+
+The Proton C only has one on-board LED (C13), and by default, the TXLED (D5) is mapped to it. If you want the RXLED (B0) mapped to it instead, add this like to your `config.h`:
+
+    #define CONVERT_TO_PROTON_C_RXLED