[Keymap] New keymap for Keebio FoldKB (#17364)

keyboards/keebio/foldkb/keymaps/forrcaho/readme.md

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+# Keebio FoldKB rev1
+
+My layout for the FoldKB has three layers:
+
+* A default standard QWERTY layer, 
+
+* a second "programming" layer with
+    * function keys for the corresponding numbers
+    * arrow and other navigation keys (such as home) on the right half, and
+    * various alternate keymappings and macros useful for programming on the
+    left half.
+
+* a third "mouse" layer with mouse keys on the right half. No special functions
+have been added to the left half for this layer (yet).
+
+This was written for a FoldKB with one encoder, and that is made to function as
+a mouse scroll wheel on all layers. Pressing it is a mouse left click and
+shift-pressing it is a mouse right click. (Presumably both encoders would do the
+same thing on a FoldKB with two.)
+
+The backgound lighting levels indicate the layer: dimmest for the default
+QWERTY layer, middle brightness for the mouse layer, and brightest for the
+programming layer.