Contributor Guide

[RobbieMcKinstry]

This PR adds a contributor's guide similar to the one pending for the Docker Container repo.

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Thank you for contributing! Before submitting your PR, can you please confirm that you have done the following?

• [ ] If you changed the values of the properties name, publisher, and galleryFlags in the vss-extension.json file, you have reverted them.
• [ ] If you changed the values of the properties id, name, author, you have reverted them.
• [ ] I have reverted any changes to the version number components in both vss-extension.json and task.json, OR I didn't change them.