Update contributing guideline

CONTRIBUTING.md → misc/CONTRIBUTING.md

@@ -3,6 +3,10 @@ Contributions to nnn are welcome! There's always an open issue with the current
 
 Before suggesting changes, please read a bit about [the design principles nnn follows](https://github.com/jarun/nnn/wiki#design), and make sure you aren't breaking any of them.
 
+## Coding standard
+
+`nnn` follows the Linux kernel coding style closely. The C source code uses TABs and the plugins use 4 spaces for indentation.
+
 ## Resources
 The [wiki](https://github.com/jarun/nnn/wiki/Developer-guides) has some resources for developers you might be interested in: building, debugging...