Enable third party contributions

- Add instructions for contributing to ARM Trusted Firmware.

- Update copyright text in all files to acknowledge contributors.

Change-Id: I9311aac81b00c6c167d2f8c889aea403b84450e5
diff --git a/Makefile b/Makefile
index c2b258f..e1b0b8f 100644
--- a/Makefile
+++ b/Makefile
@@ -1,5 +1,5 @@
 #
-# Copyright (c) 2013, ARM Limited. All rights reserved.
+# Copyright (c) 2013, ARM Limited and Contributors. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are met: