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/docs/porting-guide.md b/docs/porting-guide.md
index a3ef99e..0b07111 100644
--- a/docs/porting-guide.md
+++ b/docs/porting-guide.md
@@ -928,7 +928,7 @@
 
 - - - - - - - - - - - - - - - - - - - - - - - - - -
 
-_Copyright (c) 2013 ARM Ltd. All rights reserved._
+_Copyright (c) 2013, ARM Limited and Contributors. All rights reserved._
 
 
 [User Guide]: user-guide.md