Document Transfer List Library upstream repo

Signed-off-by: Jose Marinho <jose.marinho@arm.com>
Change-Id: I70d80f4071907a9d354cbe3b0dd0ac8f85470ee7

Author: jmarinho

README.md

@@ -44,6 +44,15 @@ make html
 
 The output of these build commands goes into subdirectory `build`.
 
+Reference implementation
+========================
+
+The specification has a library reference implementation, its hosted upstream in the following
+repository: [Transfer List Library](https://github.com/TF-Shared/transfer-list-library).
+
+The library can be leveraged by different firmware projects. It implements all the relevant
+operations on the Transfer List.
+
 Status
 ======