Add 1.1.15 release notes.

[duderino]

Please provide a description for what this PR is for.

And to help us figure out who should review this PR, please
put an X in all the areas that this PR affects.

[ ] Configuration Infrastructure
[ ] Docs
[ ] Installation
[ ] Networking
[ ] Performance and Scalability
[ ] Policies and Telemetry
[ ] Security
[ ] Test and Release
[ ] User Experience
[ ] Developer Infrastructure

[istio-testing]

@duderino: The following test failed, say /retest to rerun them all:

Test name	Commit	Details	Rerun command
lint_istio.io	1eaa51b	link	/test lint_istio.io

Details

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[duderino]

@geeknoid looks like the build failures are unrelated to this PR:

Total in 13982 ms
>> 460 files are free from spelling errors
Running ["ScriptCheck", "OpenGraphCheck", "LinkCheck", "HtmlCheck", "ImageCheck"] on ["./public"] on *.html... 


Found 0 links in the cache...
Adding 827 links to the cache...
Removing 0 links from the cache...
Checking 827 external links...
Ran on 376 files!


- ./public/blog/2019/evolving-istios-apis/index.html
  *  External link https://github.com/knative/serving/blob/master/docs/spec/spec.md#route failed: https://github.com/knative/serving/blob/master/docs/spec/spec.md exists, but the hash 'route' does not
htmlproofer 3.9.2 | Error:  HTML-Proofer found 1 failure!
LINTING FAILED
Makefile.core.mk:27: recipe for target 'lint' failed
make[1]: *** [lint] Error 1
Makefile:84: recipe for target 'lint' failed
make: *** [lint] Error 2

[istio-testing]

In response to a cherrypick label: new pull request created: #5008