Add security@gradle.com email address to CONTRIBUTING.md

[JLLeitschuh]

Signed-off-by: Jonathan Leitschuh Jonathan.Leitschuh@plexxi.com

Context

The security@gradle.com email address isn't publicized anywhere. If you do a google search for the string "security@gradle.com" in quotes google returns zero results. The only way I found out about it was by asking around to the various Gradle Team members on slack.

You should also consider finding a place on the https://gradle.org/ site so that it can be found/indexed better by google.

Contributor Checklist

• Review Contribution Guidelines
• Make sure that all commits are signed off to indicate that you agree to the terms of Developer Certificate of Origin.
• Check "Allow edit from maintainers" option in pull request so that additional changes can be pushed by Gradle team
• Provide integration tests (under <subproject>/src/integTest) to verify changes from a user perspective
• Provide unit tests (under <subproject>/src/test) to verify logic
• Update User Guide, DSL Reference, and Javadoc for public-facing changes
• Ensure that tests pass locally: ./gradlew <changed-subproject>:check

Gradle Core Team Checklist

• Verify design and implementation
• Verify test coverage and CI build status
• Verify documentation
• Recognize contributor in release notes