docs: Add upcoming Change Log to Table of Contents

Signed-off-by: Lauren Wehrmeister <lauren.wehrmeister@arm.com>
Change-Id: I2a7f38eaae3a78fc3caa37833af755c15e8236ce

docs/index.rst

@@ -15,6 +15,7 @@ Trusted Firmware-A Documentation
    perf/index
    security_advisories/index
    change-log
+   change-log-upcoming
    glossary
    license