adds documentation for console in

docs/feature_console_in.md

@@ -0,0 +1,26 @@
+# Console In
+
+The "Console" in QMK refers to PJRC's vendor page of `0xFF31` and usage page of `0x74` - this is mainly used for sending debug statements to the computer that can be read via the QMK Toolbox (recommended), or HID listen.
+
+When `#define CONSOLE_IN_ENABLE` is placed in the `config.h` file, the keyboard will also listen to messages sent to the keyboard, will be able to respond in a custom way, defined at the `quantum`, `kb`, or `user` level. A keymap's example might look like this:
+
+```c
+void process_console_data_user(uint8_t * data, uint8_t length) {
+  switch (data[0]) {
+    case 0x05:
+      print("Sending back a message to the computer\n");
+      break;
+  }
+}
+```
+
+Which will execute when a report is sent to the keyboard when the first byte is `0x05`.
+
+## Default actions (defined at the `quantum` level)
+
+When the first byte is:
+
+* `0x01`: Print "Saying hello\n" via the console, and play the audio on song
+* `0xFE`: Enter the bootloader - only works when `CONSOLE_IN_BOOTLOADER` is defined in the `config.h`
+
+This needs to be fleshed out a bit more with some sort of versioning.