doc: add security.md to the repo

[prmathur-microsoft]

Checklist

• I have read the [contribution guidelines] (https://github.com/Azure/azure-iot-sdk-java/blob/master/.github/CONTRIBUTING.md).
• I added or modified the existing tests to cover the change (we do not allow our test coverage to go down).
• If this is a modification that impacts the behavior of a public API
  • I edited the corresponding document in the devdoc folder and added or modified requirements.
• I submitted this PR against the correct branch:
  • This pull-request is submitted against the master branch.

Reference/Link to the issue solved with this PR (if any)

Description of the problem

Description of the solution

[prmathur-microsoft]

/azp run

[azure-pipelines]

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