Add information on the practice mode to the readme

readme.md

@@ -17,6 +17,13 @@ required for your OS version.
 Use the arrow keys to move, 2nd to jump and alpha to dash. To exit the game, press
 the clear key to save your progress, or del to exit without saving.
 
+A practice mode for speedrunning can be accessed by pressing the graph key from
+the title screen. In this mode, the time for the current room is displayed in white, and
+your best time for the room (in frames) is displayed in yellow. After completing a room,
+you will restart on the same level. To move to the previous level, press y=; to move to
+the next, press graph. You can also press window to toggle the HUD, and zoom to toggle
+gemskip mode.
+
 ### Compiling
 To build this repository from source, the [CE C Toolchain](https://github.com/CE-Programming/toolchain)
 is required. After installing the toolchain, clone this repository and run `make gfx`