Adding contributing guide and contributors list. (#570)

Adding contributing guide.

.git.commit.template

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+Summary of change in one line.
+
+Motivation:
+
+Why is this change being made?
+
+Modifications:
+
+- List the changes
+
+Result:
+
+What is the result of this change?

CONTRIBUTING.adoc

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+= Contributing to ServiceTalk
+
+Welcome to the ServiceTalk community, and thank you for contributing! The following guide outlines the basics of how to
+get involved. Pull requests to update and expand this guide are welcome.
+
+==== Table of Contents
+
+* <<Before you get started>>
+** <<Community Guidelines>>
+** <<Project Licensing>>
+** <<Governance and Decision Making>>
+* <<Contributing>>
+** <<Opening a Pull Request>>
+** <<Reporting Issues>>
+* <<Project Communication>>
+
+== Before you get started
+=== Community Guidelines
+We want the ServiceTalk community to be as welcoming and inclusive as possible, and have adopted a link:CODE_OF_CONDUCT.md[Code of Conduct]
+that we ask all community members to read and observe.
+
+=== Project Licensing
+By submitting a pull request, you represent that you have the right to license your contribution to Apple and the
+community, and agree by submitting the patch that your contributions are licensed under the Apache 2.0 license.
+
+=== Governance and Decision Making
+At project launch, ServiceTalk has a light governance structure. The intention is for the community to evolve quickly
+and adopt additional processes as participation grows. Stay tuned, and stay engaged! Your feedback is welcome.
+
+Members of the Apple ServiceTalk team are part of the initial core committers helping review individual contributions;
+you'll see them commenting on your pull requests. Future committers to the open source project, and the process for
+adding individuals in this role will be formalized in the future.
+
+== Contributing
+=== Opening a Pull Request
+We love pull requests! For minor changes, feel free to open up a PR directly. For larger feature development and any
+changes that may require community discussion, we ask that you discuss your ideas on a link:https://github.com/servicetalk/servicetalk/issues[github issue]
+prior to opening a PR, and then reference that issue within your PR comment.
+
+CI will run tests against the PR and post the status back to github.
+
+=== Writing a Patch
+A good ServiceTalk patch is:
+
+- Concise, and contains as few changes as needed to achieve the end result.
+- Tested, ensuring that any tests provided failed before the patch and pass after it.
+- Documented, adding API documentation as needed to cover new functions and properties.
+- Accompanied by a great commit message, using our <<Commit Message Template>>.
+
+=== Checklist
+Please use the following checklist before pushing your commits or issuing a pull request:
+
+- Did you rebase your pull request against the HEAD of the target branch and fix all conflicts?
+- Does your work build without any failure when you run `./gradlew build` from shell?
+- Does your commit message or pull request description follow our <<Commit Message Template>>?
+
+=== Commit Message Template
+We require that your commit messages match our template. The easiest way to do that is to get git
+to help you by explicitly using the template. To do that, `cd` to the root of our repository and run:
+```
+git config commit.template .git.commit.template
+```
+
+=== Reporting Issues
+Issues may be reported through link:https://github.com/servicetalk/servicetalk/issues[github issues].
+
+Please be sure to include:
+
+* The ServiceTalk version
+* The OS version and the output of `uname -a`
+* Java version, output of `java -version`
+* Contextual information (e.g. what you were trying to achieve with ServiceTalk)
+* Simplest possible steps to reproduce
+** A pull request with a failing test case is preferred, but it's fine to paste the test case into the issue description.
+* Anything else that might be relevant in your opinion, such as network configuration.
+
+==== Security issues
+To report a security issue, please DO NOT start by filing a public issue; instead send a
+private email to link:mailto:servicetalk-security@group.apple.com[servicetalk-security@group.apple.com].
+
+== Project Communication
+We encourage your participation asking questions and helping improve the ServiceTalk project.
+link:https://github.com/servicetalk/servicetalk/issues[Github issues] and
+link:https://github.com/servicetalk/servicetalk/pulls[pull requests] are the primary mechanisms of
+participation and communication for ServiceTalk.