Update local build README

Author: AWSstangers

local_builds/README.md

@@ -23,7 +23,8 @@ usage: codebuild_build.sh [-i image_name] [-a artifact_output_directory] [option
   -a        Used to specify an artifact output directory.  
 
 **Optional:**  
-  -s        Used to specify a source directory. Defaults to the current working directory.  
+  -s        Used to specify a source directory. Defaults to the current working directory.
+  -c        Use the AWS configuration and credentials from your local host. This includes ~/.aws and any AWS_* environment variables.
   -b        Used to specify a buildspec override file. Defaults to buildspec.yml in the source directory.  
   -e        Used to specify a file containing environment variables.