docs: Add a layouts/README.md explaining files

Author: ids1024

layouts/README.md

@@ -0,0 +1,11 @@
+`picker.json` defines the groups of keycodes that appear in the keycode picker. The labels here are used both in the picker and on the keyboard.
+
+Within each layout:
+* `default.json` - The default keymap and LED settings, in the same format the Configurator can import/export through its UI.
+* `keymap.json` - Maps keycode names to their numerical values.
+* `layout.json` - Maps key position to electrical matrix indices.
+* `leds.json` - For a keyboard with per-key LEDs, maps key position to LED index.
+* `meta.json` - Micellanous values associated with the keyboard.
+* `physical.json` - Defines the physical layout of keys, the colors to display as their backgrounds, and labels (only shown in a tab when `--debug-layers` is passed to the Configurator).
+
+Other than `meta.json` and `physical.json`, these files are generated from the EC/QMK source using `layouts.py` from the root of this repository. `meta.json` is written manually, with other keys added by `layouts.py`. `physical.json` is created with <http://www.keyboard-layout-editor.com>.