Add CONTRIBUTING.md

Author: Alexhuszagh

CONTRIBUTING.md

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+Thank you for looking to contribute to cross. Have a new feature you'd like to add? Know how to fix an open bug? Want to add an image for a new target? We host documentation for how to contribute on our [wiki](https://github.com/cross-rs/cross/wiki/Contributing).
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+Please read our [code of conduct](https://github.com/cross-rs/cross/blob/main/CODE_OF_CONDUCT.md) so our community stays positive and welcoming. If you have any additional questions, please feel free to ask in either our [discussions](https://github.com/cross-rs/cross/discussions) or our [Matrix room](https://matrix.to/#/#cross-rs:matrix.org).
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+<!--
+NOTE: we don't have specific information provided here since this
+would require us to check the contributing changes into our Git
+history and therefore CI, which would make any changes to the
+repository require separate commits or additional changes to the
+code base, making either more difficult for new contributors or
+adding unnecessary commits.
+-->