docs(MADEWITHVRTK): add Richie's Plank Experience to list

Richie's Plank Experience has been added to the list.

Author: thestonefox

MADEWITHVRTK.md

@@ -12,6 +12,7 @@ A collection of titles that have been made with VRTK.
 | Stage Presence | [Steam Store Page](http://store.steampowered.com/app/391640) |
 | Nock: Hidden Arrow | [Steam Store Page](http://store.steampowered.com/app/525210) |
 | ViveSpray | [Steam Store Page](http://store.steampowered.com/app/494830) |
+| Richie's Plank Experience | [Steam Store Page](http://store.steampowered.com/app/517160) |
 | VeeR Pong | [Steam Store Page](http://store.steampowered.com/app/494850) |
 | Emergence Fractal Universe | [Steam Store Page](http://store.steampowered.com/app/500470) |
 | Tower Island: Explore, Discover and Disassemble | [Steam Store Page](http://store.steampowered.com/app/487740) |