More detailed readme, move image to img folder

readme.md

@@ -6,18 +6,17 @@ It is ported from my QMK configuration, which in turn is heavily inspired by Man
 
 ## Key features
 
-- simple macro-implementation of combos and keymap to fit different physical keyboards
+- simple macro-implementation of combos and keymap to fit different physical
+  keyboards[^1]
 - home-row mods on base layer, sticky mods on `Nav` and `Num` layers
 - most symbols can be accessed from the base layer via combos
 - sticky shift on right thumb, double-tap activates caps-word
 - backspace morphs into delete when shifted
 - unicode layer with Greek letters for mathematical typesetting (not yet ported to ZMK)
 
-## Keymap layout
+![](img/keymap.png)
 
-![](keymap.png)
-
-## Thoughts on combo layout
+## A few thoughts on the combo setup
 
 The combo layout is guided by two goals: (1) put all combos in easy-to-access locations,
 and (2) make them easy to remember. Specifically:
@@ -25,12 +24,24 @@ and (2) make them easy to remember. Specifically:
 - the top vertical-combo row is almost equivalent to the symbols on standard number rows,
   making them easy to remember
 - the bottom vertical-combo row is set up symmetrically to facilitate memorization
-  (subscript `_` aligns with superscript `^`; `-` aligns with `+`; division `/` aligns 
-  with multiplication `*`; logical-or aligns with logical-and; back-slash aligns 
-  horizontally with forward-slash)
+  (subscript `_` aligns with superscript `^`; minus `-` aligns with `+`; division `/`
+  aligns with multiplication `*`; logical-or `|` aligns with logical-and `&`; backslash
+  `\` aligns horizontally with forward slash `/`)
 - parenthesis, brackets, braces and punctuation are on the right-hand side for easy
   usage when typing numbers
 - a numlock shortcut (on `W + P`) is on the left-hand side for one-handed number-mode
-- shortcuts for cut (on `X+D`), copy and paste are on the left-hand side for one-handed
-  usage when using the mouse
+- shortcuts for cut (on `X + D`), copy, and paste are on the left-hand side for
+  one-handed usage when using the mouse
 
+[^1]: I use a 36-key layout per default. Additional thumb keys can be configured with
+    the `EXTRA_BOT_L` and `EXTRA_BOT_R` macros. Additional "middle-keys" can be
+    configured with the `EXTRA_MID` macro (see `planck_rev6.keymap` for an example).
+
+    In order to consitently configure combos and [positional
+    hold-taps](https://zmk.dev/docs/behaviors/hold-tap#positional-hold-tap-and-hold-trigger-key-positions)
+    across keyboards with different physical key specifications, I use a set of "virtual
+    location" macros which map key-positions in the layout to physical locations on the
+    shield. These macros follow a common naming convention, starting with `LT0` for the
+    first key on the **L**eft **T**op-row and ranging to `RB4` for the last key on the
+    **R**ight **B**ottom-row. They need to be defined in the shield-specific keymap-file
+    *before* sourcing the common `base.keymap`.