Split atreus (#1680)

* Merge with upstream

* Add split_atreus keymap for lets_split.

* Move bspace

* Tidy up keymap

* Update layer enum

* Temporary fix

* Finish merge

* Added keys for testing

* Revert temporary fix

* Remake keymap using default let's split map as base. ASCII art not
updated

* Remove non-functioning layers

* Rename folder

* Update ASCII art

* Remove vestiges of ADJUST layer

* Remove nonexistent layer #defines

* Rename folder to avoid confusion

* Add Readme.md

* Formatting

* Description of underscores in layer maps

keyboards/lets_split/keymaps/henxing/Readme.md

@@ -0,0 +1,51 @@
+# Let's Split the Atreus
+
+This keymap is a port of the Atreus keymap to the Let's Split keyboard, using a
+similar method of handling layer switches as the `default` keymap.
+
+
+# Layers
+
+The following tables are the layers as implemented.
+
+### Underscores in Layer
+
+Any underscore represents a key that is mapped to `KC_NO`, or no keypress. The
+2x3 block in the middle is like this because the Atreus does not have any keys
+there. It has been left blank for the user to add her own special keypresses or
+macros that she finds useful to have on that particular layer.
+
+
+## `_QWERTY`
+
+```c
+/*
+ *  q   w   e    r    t    _    _    y   u   i  o   p
+ *  a   s   d    f    g    _    _    h   j   k  l   ;
+ *  z   x   c    v    b    _    _    n   m   ,  .   /
+ * esc tab gui shift bksp ctrl alt space fn  -  ' enter
+ */
+ ```
+
+
+## `_LOWER`
+
+```c
+/*
+ *  !       @     up     {    }    _     _  pgup    7     8     9    *
+ *  #     left   down  right  $    _     _  pgdn    4     5     6    +
+ *  [       ]      (     )    &    _     _    `     1     2     3    \
+ * lower  insert  gui  shift bksp ctrl  alt space   fn    .     0    =
+ */
+ ```
+
+## `_RAISE`
+
+```c
+/*
+ * insert home   up   end  pgup   _     _   up     F7    F8    F9   F10
+ *  del   left  down right pgdn   _     _  down    F4    F5    F6   F11
+ *   _   volup    _    _   reset  _     _          F1    F2    F3   F12
+ *   _   voldn super shift bksp  ctrl  alt space   L0  prtsc scroll pause
+ */
+ ```