Add slides for Gerrit 2.15 talk

Change-Id: I10e029705261feb4dea7c6764852549570b883e8

sessions/new-in-2.15.md

diff --git a/sessions/new-in-2.15.md b/sessions/new-in-2.15.md
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--- a/sessions/new-in-2.15.md
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@@ -2,4 +2,6 @@
 
 Preview of the new features coming in the next release of Gerrit.
 
+[Slides](https://storage.googleapis.com/gerrit-talks/summit/2017/Whats-New-in-Gerrit-2.15.pdf)
+
 *[Dave Borowitz, Google](../speakers.md#dborowitz)*