docs: add gitpod badge

README.md

@@ -8,3 +8,5 @@ It also contains a motivation to consider such an object.
 This example was presented during:
 * the exercise classes to [_Aspects of Nonlinear Wave Equations_](https://www.math.kit.edu/iana2/edu/nonlinwave2016s/) (summer semester 2016, Karlsruhe),
 * the talk [_O pewnej funkcji nigdzie wypukłej z wypukłym ograniczeniem górnym_](http://knm.katowice.pl/spotkanie_21.12.2016_piotr_idzik.php) (in Polish) [[_About a certain nowhere convex function with a convex upper bound_](http://en.knm.katowice.pl/spotkanie_21.12.2016_piotr_idzik.php)] (Katowice, 21.12.2016).
+
+[![Open in Gitpod](https://gitpod.io/button/open-in-gitpod.svg)](https://gitpod.io/#https://github.com/vil02/nowhere_convex)