Rephrase "TL;DR"

"TL;DR" seems not to be understood or respected by everyone. Maybe we should do a try with "Important". Open for alternative suggestions.

CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 If you have questions about how to install or use ownCloud, please direct these to the [mailing list][mailinglist] or our [forum][forum]. We are also available on [IRC][irc].
 
-### TL;DR
+### Important: In a nutshell
 
  * The [**issue template can be found here**][template] but be aware of the different repositories! See list below.