Add documentation for Combo feature

docs/config_options.md

@@ -155,6 +155,10 @@ If you define these options you will enable the associated feature, which may in
     going to produce the 500 keystrokes a second needed to actually get more than a
     few ms of delay from this. But if you're doing chording on something with 3-4ms
     scan times? You probably want this.
+* `#define COMBO_COUNT 2`
+  * Set this to the number of combos that you're using in the [Combo](feature_combo.md) feature.
+* `#define COMBO_TERM 200`
+  * how long for the Combo keys to be detected. Defaults to `TAPPING_TERM` if not defined.
 
 ## RGB Light Configuration
 
@@ -234,6 +238,8 @@ Use these to enable or disable building certain features. The more you have enab
   * Console for debug(+400)
 * `COMMAND_ENABLE`
   * Commands for debug and configuration
+* `COMBO_ENABLE`
+  * Key combo feature
 * `NKRO_ENABLE`
   * USB N-Key Rollover - if this doesn't work, see here: https://github.com/tmk/tmk_keyboard/wiki/FAQ#nkro-doesnt-work
 * `AUDIO_ENABLE`