Add nim playground to playground list (#6118)

* Add nim playground to playground list

* Add correct spacing between headers

* Add organisation to link

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

Co-authored-by: David Ordás <3125580+davorpa@users.noreply.github.com>

more/free-programming-playgrounds.md

@@ -21,6 +21,7 @@
 * [Kotlin](#kotlin)
 * [Kubernetes](#kubernetes)
 * [.Net](#dotnet)
+* [Nim](#nim)
 * [Node.js](#nodejs)
 * [OCaml](#ocaml)
 * [Perl](#perl)
@@ -171,6 +172,11 @@
 * [.NET Fiddle](https://dotnetfiddle.net)
 
 
+### Nim
+
+* [Nim Playground](https://play.nim-lang.org) - nim-lang.org
+
+
 ### NodeJS
 
 * [Ideone](http://ideone.com)