Updated FAQ Keymap (markdown)

FAQ-Keymap.md

@@ -1,4 +1,8 @@
 # Keymap
+## How to get keycode
+See [doc/keycode.txt](https://github.com/tmk/tmk_keyboard/blob/master/doc/keycode.txt).
+Keycodes are actually defined in [common/keycode.h](https://github.com/tmk/tmk_keyboard/blob/master/common/keycode.h).
+
 ## Sysrq key
 Use keycode for Print Screen(`KC_PSCREEN` or `KC_PSCR`) instead of `KC_SYSREQ`. Key combination of 'Alt + Print Screen' is recognized as 'System request'.