Add 1.1.15 release notes. (#5005)

content/en/about/notes/1.1.15/index.md

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+---
+title: Istio 1.1.15
+publishdate: 2019-09-16
+icon: notes
+release: 1.1.15
+---
+
+This release includes bug fixes to improve robustness.  This release note describes what's different between Istio 1.1.14 and Istio 1.1.15.
+
+{{< relnote >}}

content/en/blog/2019/announcing-1.1.15/index.md

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+---
+title: Announcing Istio 1.1.15
+description: Istio 1.1.15 patch release.
+publishdate: 2019-09-16
+attribution: The Istio Team
+release: 1.1.15
+---
+
+We're pleased to announce the availability of Istio 1.1.15. Please see below for what's changed.
+
+{{< relnote >}}

content/en/boilerplates/notes/1.1.15.md

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+## Bug fixes
+
+- Fix an Envoy crash introduced in Istio 1.1.14 ([Issue 16357](https://github.com/istio/istio/issues/16357)).
+
+## Small enhancements
+
+- Expose `HTTP/2` window size settings as Pilot environment variables ([Issue 17117](https://github.com/istio/istio/issues/17117)).