chore(community): add contributors file (#18)

CONTRIBUTORS.md

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+Contributors of uiGradients
+============================
+uiGradients is developed and maintained by the open source community. You're more than welcome to contribute and make feature proposals, provided you upkeep the [Code of Conduct](CODE_OF_CONDUCT.md).
+
+Supporters
+----------
+uiGradients is supported by [JSBro](http://jsb.ro).
+
+Maintainers
+----------
+Committers are people who have made substantial contribution to the project and granted write access to the project.
+* Garet McKinley [@garetmckinley](https://github.com/garetmckinley)
+* Matt Hamil [@matthamil](https://github.com/matthamil)
+
+Become a Maintainer
+------------------
+Maintainers are contributors that have shown leadership skills and a strong sense of community.
+
+* Made substantial contribution to the project.
+* Willing to spent time on maintaining and lead the project.
+* Compliance with our [Code of Conduct](CODE_OF_CONDUCT.md).
+
+New maintainers will be proposed by current maintainer, with support from more than one other maintainer. If you're interested in becoming a maintainer, it is highly recommended to [join our slack room](https://slackin-xtuseyimsc.now.sh/) (open to anybody).
+
+List of Contributors
+--------------------
+* Open a pull request to become a contributor!