[Docs] Add CONTRIBUTING.md to solicit contributions

-Add info on ways to contribute
-Add details on how to set up dev environment
-Add info on how to submits PRs
-Include a table of contents with links to document headings

Fixes #845

CONTRIBUTING.md

@@ -0,0 +1,6 @@
+
+# Contributing to ``eslint-plugin-jsx-a11y``
+
+Thank you for your interest in ``eslint-plugin-jsx-a11y``. [Here](https://github.com/jsx-eslint/eslint-plugin-jsx-a11y/issues?q=is%3Aissue+is%3Aopen+label%3A%22help+wanted%22) you can find the issues that are ready for contributions.
+
+ Please refer to the [following](https://github.com/jsx-eslint/.github/blob/HEAD/CONTRIBUTING.md) for further information.