Adding release notes for helix 0.9.9

[zhangmeng916]

Issues

• My PR addresses the following Helix issues and references them in the PR description:

NA
only doc change

Description

• Here are some details about my PR, including screenshots of any UI changes:

Adding release notes for 0.9.9

Tests

• The following is the result of the "mvn test" command on the appropriate module:

mvn site:site
mvn site:stage

Documentation (Optional)

• In case of new functionality, my PR adds documentation in the following wiki page:

(Link the GitHub wiki you added)

Commits

• My commits all reference appropriate Apache Helix GitHub issues in their subject lines. In addition, my commits follow the guidelines from "How to write a good git commit message":
  1. Subject is separated from body by a blank line
  2. Subject is limited to 50 characters (not including Jira issue reference)
  3. Subject does not end with a period
  4. Subject uses the imperative mood ("add", not "adding")
  5. Body wraps at 72 characters
  6. Body explains "what" and "why", not "how"

Code Quality

• My diff has been formatted using helix-style.xml
  (helix-style-intellij.xml if IntelliJ IDE is used)

[jiajunwang]

One more comment, do we have the tutorial for the cloud support features?

[junkaixue]

Let's wait for the new domain API.

[narendly]

I believe there was a recent contribution to the tutorial. Can you make sure that change has been applied to the tutorial for this version?