Improve documentation

README.md

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 Desktop version of [xkcd 1608 "Hoverboard"](https://xkcd.com/1608/) game.
 
+## Why
+
+I'm a big fan of [xkcd](https://xkcd.com/) by Randall Munroe, and
+now he has released an exploration game to celebrate [his new
+book](https://xkcd.com/thing-explainer/). The game is great as all
+his comics, however it unfortunately is only available on web, which
+implies:
+
+* You can't play offline
+* It's a pain to play with poor or intermittent connection
+* Inefficient web technologies waste your battery life and may produce noticeable lags
+* You are limited with fixed 740x700 visible area, which may take only a small part of available screen space, or, vice versa, not fit into the screen
+* You can't see the source
+* You can't improve it, for example implementing save/load or a minimap
+
+So there's plenty of reasons to have a proper native application,
+and, given that Randall makes the art available under [CC-by-NC
+license](https://xkcd.com/license.html), I want to accomplish that.
+
+## Roadmap
+
+- [x] Proof of concept - just a scrollable map
+- [ ] Game elements - player, physics, coins
+- [ ] Additional improvements such as mentioned save/load
+
 ## Building
 
 Dependencies:
@@ -30,7 +55,7 @@ cmake . && make
 ## License
 
 * Code: GPLv3 or later, see COPYING
-* Data: Creative Commons Attribution-NonCommercial 2.5 License, see [kxcd.com](https://xkcd.com/license.html).
+* Assets by Randall Munroe, Creative Commons Attribution-NonCommercial 2.5 License, see [kxcd.com](https://xkcd.com/license.html).
 
 The project also bundles third party software under its own licenses: