AStyle: add .astylerc options file

Author: 0xc0170

docs/reference/contributing/guidelines/style.md

@@ -159,9 +159,11 @@ All functions and methods should contain documentation using Doxgyen.
 You can use [Artistic Style (AStyle)](http://sourceforge.net/projects/astyle/files/) to format your code. Use the command-line switch to select the correct style and point to the file you want to edit:
 
 ```
-astyle --style=kr --indent=spaces=4 --indents-switches $(full_path_to_file)
+astyle -n --options=.astylerc $(full_path_to_file)
 ```
 
+File `.astylerc` defines Mbed OS code style and it's located in Mbed OS root directory.
+
 #### Compiler settings
 
 All C and C++ code submitted to Mbed OS must compile with GCC Arm Embedded, Arm Compiler 5 and IAR EWARM. Mbed OS: