Initial potentiometer support

docs/feature_potentiometers.md

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+# Potentiometers
+
+Add this to your `rules.mk`:
+
+```make
+POTENTIOMETER_ENABLE = yes
+```
+
+and this to your `config.h`:
+
+```c
+#define POTENTIOMETER_PINS { B0 }
+```
+
+## Callbacks
+
+The callback functions can be inserted into your `<keyboard>.c`:
+
+```c
+bool potentiometer_update_kb(uint8_t index, uint16_t value) {
+    if (!potentiometer_update_user(index, value)) {
+        midi_send_cc(&midi_device, 2, 0x3E, 0x7F + value);
+    }
+    return true;
+}
+```
+
+or `keymap.c`:
+
+```c
+bool potentiometer_update_user(uint8_t index, uint16_t value) {
+    midi_send_cc(&midi_device, 2, 0x3E, 0x7F + value);
+    return false;
+}
+```