[Keyboard] Add Signum 3.0 for the elite-c (#5478)

* Add keyboard Signum 3.0 for the elite-c

* Update readme.md

* Remove unused macros

* Use bootmagic lite instead of full

* Add warning to keymap.c that it's auto-generate

* Add description for customizing keyboard layouts

* Make generate_km.py executable

* Make right shift available in numpad layer

* Update keyboards/signum/3_0/elitec/readme.md

Co-Authored-By: jceb <jceb@e-jc.de>

* Update keyboards/signum/3_0/elitec/readme.md

Co-Authored-By: jceb <jceb@e-jc.de>

* Fix typo in symbol name

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

@@ -0,0 +1,27 @@
+# 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
+
+- Install `python3` and [Sorted Containers
+  Library](http://www.grantjenks.com/docs/sortedcontainers/), `pip
+  install sortedcontainers`.
+
+- 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`.