Add clang format file

This is a slightly modified version of Ikey's format for usysconf found here: https://github.com/solus-project/usysconf/blob/master/.clang-format

Modifications are to change to tab indentation, and tab/indent widths of 4
You'll never please everyone, but I find tabs for indentation is the most pragmatic approach to keeping most people happy about indent length arguments.

README.md

@@ -104,6 +104,13 @@ Add `-lgamemodeauto` to linker arguments and distribute `libgamemodeauto.so` wit
 #### Option 3: Distribute and script
 Distribute `libgamemodeauto.so` with the game and add to LD\_PRELOAD in a launch script
 
+---
+## Pull Requests
+Pull requests must match with the coding style found in the `.clang-format` file
+```
+clang-format -i $(find . -name '*.[ch]')
+```
+
 ---
 ## TODO