README: Fix documentation regarding config files

Signed-off-by: Michal Hrusecky <Michal@Hrusecky.net>

README.asciidoc

@@ -26,7 +26,8 @@ directories, patches are also divided into subdirectories like `to-upstream`,
 Configuration
 ~~~~~~~~~~~~~
 
-Default config for OpenWRT is put in `configs` directory either in file
-`common` or in a file named after the target board. Those two files gets merged
-and will form the final `.config` file. Some parts of `compile_fw` might modify
-it further.
+Default config for OpenWRT is put in `configs` directory either in directory
+`common` or in a directory named after the target board. Files in those
+directories gets merged (common ones first) and will form the final `.config`
+file. Some parts of `compile_fw` might modify it further.
+