Doc: document security incident reporting process

Add a reference to the tf.org Security Center where the incident
reporting process is described.

Change-Id: Icff9e3ffae2f109c35f1cca5d685f3f360e0a194
Signed-off-by: Gyorgy Szing <Gyorgy.Szing@arm.com>
diff --git a/docs/project/contributing.rst b/docs/project/contributing.rst
index 836502c..d704c24 100644
--- a/docs/project/contributing.rst
+++ b/docs/project/contributing.rst
@@ -1,6 +1,10 @@
 Contributing
 ============
 
+Reporting Security Issues
+-------------------------
+
+Please follow the directions of the `Trusted Firmware Security Center`_
 
 Getting Started
 ---------------
@@ -81,7 +85,7 @@
 .. _Gerrit Signed-off-by Lines guidelines: https://review.trustedfirmware.org/Documentation/user-signedoffby.html
 .. _Gerrit Change-Ids documentation: https://review.trustedfirmware.org/Documentation/user-changeid.html
 .. _`Tim Popes blog entry`: https://tbaggery.com/2008/04/19/a-note-about-git-commit-messages.html
-
+.. _`Trusted Firmware Security Center`: https://developer.trustedfirmware.org/w/collaboration/security_center/
 
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