[Keymap] moonlander:shahn-neo (#10667)

* basic keys working

* more stuff

* more stuff

* empty upper layers

* more stuff

* cleanup

* more stuff

* add the letter 'j'

* add '0' to layer 4

* implement umlauts and eszett with menu key as compose key

* add comment

* renamed layout

* change copyright notice

* add separate readme file

keyboards/moonlander/keymaps/shahn-neo/readme.md

@@ -0,0 +1,16 @@
+# Sönke Hahn's Neo layout for the Moonlander keyboard
+
+This is my personal keyboard layout based on the Neo keyboard layout:
+https://en.wikipedia.org/wiki/Neo_(keyboard_layout)
+This layout is not meant as a general layout for other people using the
+Neo keyboard layout. It's very specific to my personal needs.
+It still might be useful as inspiration to other people who want to
+create a Neo layout.
+
+Some notes:
+
+- I've put the ctrl key, the shift keys and the modifiers to switch to
+  layer 4 in the middle to be operated by the thumbs. I wanted to avoid
+  having to use my pinkies on the outer rows of the keyboard.
+- The umlauts (äöü) and the ß only work on linux when configuring the
+  compose key properly. See `compose_key` in `./keymap.c`.