[Keyboard] refactor signum30 (#14527)

Co-authored-by: Drashna Jaelre <drashna@live.com>
Co-authored-by: Ryan <fauxpark@gmail.com>
Co-authored-by: Stan Gurenkov <stan.gurenkov@docusign.com>

keyboards/signum/3_0/keymaps/default/readme.md

@@ -0,0 +1,24 @@
+# Default layout
+
+Layout created by: [Jan Christoph Ebersbach](https://github.com/jceb)
+
+The `generate_km.py` file automatically generates `keymap.c` with
+additional comments that visually represent the layout.
+
+This layout is maintained as I am using it. You can also use it as a
+starting point to create your own layouts for the Signum 3.0 with
+`km_template.txt` and `generate_km.py`.
+
+# Customization
+
+- Customize `layout.py` to your liking.
+    - Attention 1: keycodes are either translated into symbols and
+      function calls via the `translate()` function in `generate_km.py`
+      or via user-defined mappings in the dictionary `qmk_dict` in
+      `layout.py`. Especially the latter might require adjustment for
+      uncommon qmk features
+    - Attention 2: additional C functions need to be put into
+      `km_template.txt`
+
+Run `./generate_km.py` to regenerate `keymap.c` and the JSON keymap
+definition that should be copied to `qmk_configurator/public/keymaps`.