Update main image to better handle dark background (#1883)

Fixes #1880

Signed-off-by: Marc Khouzam <[email protected]>
Co-authored-by: Deleplace <[email protected]>

Author: marckhouzam

README.md

@@ -1,4 +1,4 @@
-![cobra logo](https://cloud.githubusercontent.com/assets/173412/10886352/ad566232-814f-11e5-9cd0-aa101788c117.png)
+![cobra logo](assets/CobraMain.png)
 
 Cobra is a library for creating powerful modern CLI applications.