Docs added

docs/layers.md

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+# Layers
+Layers extention adds keys for accessing other layers. It can simply be added to the extentions list.
+
+```python
+from kmk.extensions.layers import Layers
+keyboard.extensions.append(Layers())
+```
+
+ ## Keycodes
+
+|Key         |Description                                                                  |
+|-----------------|------------------------------------------------------------------------|
+|`KC.DF(layer)`      |Switches the default layer                                           |
+|`KC.MO(layer)`      |Momentarily activates layer, switches off when you let go            |
+|`KC.LM(layer, mod)` |As `MO(layer)` but with `mod` active                                 |
+|`KC.LT(layer, kc)`  |Momentarily activates layer if held, sends kc if tapped              |
+|`KC.TG(layer)`      |Toggles the layer (enables it if no active, and vise versa)          |
+|`KC.TO(layer)`      |Activates layer and deactivates all other layers                     |
+|`KC.TT(layer)`      |Momentarily activates layer if held, toggles it if tapped repeatedly |
+