CONTRIBUTING.md: add this

Mainly to get some text recommending rustfmt-on-save for pull requests.

CONTRIBUTING.md

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+# Contributing to Tectonic
+
+Thanks for your interest in contributing to Tectonic!
+
+There are a variety of ways to contribute to the project. This document is
+concerned with things that you do on GitHub: submitting code and managing bug
+reports. Please see
+[the corresponding section of the main Tectonic website](https://tectonic-typesetting.github.io/contribute.html)
+for a bigger-picture overview.
+
+## Code of Conduct
+
+The only restriction for contributions is that you must abide by Tectonic’s
+[Code of Conduct](./CODE_OF_CONDUCT.md). In summary, it says that you should
+not act like a jerk. Good-faith efforts to follow the letter and spirit of the
+Code of Conduct are *required* of all Tectonic community members.
+
+## Pull Requests
+
+Pull requests are very welcome! If you are pondering a substantial change,
+please
+[open an issue](https://github.com/tectonic-typesetting/tectonic/issues/new)
+before starting work so that the design can be discussed with the development
+team.
+
+The [CI](https://en.wikipedia.org/wiki/Continuous_integration) system requires
+that all submitted Rust code be formatted according to the
+[rustfmt](https://github.com/rust-lang/rustfmt#readme) tool. We *strongly*
+recommend that you achieve this by configuring your text editor to apply
+`rustfmt` formatting every time you save a file,
+[as per these instructions](https://github.com/rust-lang/rustfmt#running-rustfmt-from-your-editor).
+The Git history stays easier to read this way since we avoid making commits
+that simply reformat existing code.
+
+## Legalities
+
+New Tectonic code is licensed under the MIT License, and we assign its copyright
+to “the Tectonic Project”. Any code that you submit to Tectonic will be assumed
+to be made available under these terms unless explicitly stated otherwise.