Add initial CONTRIBUTING

CONTRIBUTING.md

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+# Contributing
+
+Thanks for considering helping this project.  There are many
+ways you can help: using the library and reporting bugs,
+reporting usability issues, making additions and improvements
+to the library, documentation and finding security bugs.
+
+## Reporting bugs
+
+Please file a github issue.  Include as much information as
+possible.  Suspected protocol bugs are easier debugged with
+a pcap or reproduction steps.
+
+Feel free to file github issues to get help, or ask a question.
+
+## Code changes
+
+Some ideas and guidelines for contributions:
+
+- For large features, file an issue prior to starting work.
+  This means everyone can see what is in progress prior to a PR.
+- Feel free to submit a PR even if the work is not totally finished,
+  for feedback or to hand-over.
+- Prefer not to reference github issue or PR numbers in commits.
+- Try to keep code formatting commits separate from functional commits.
+- See [`.travis.yml`](.travis.yml) for how to run the various test
+  suites, and how to make coverage measurements.
+- I run `cargo outdated` prior to major releases; but PRs to update specific
+  dependencies are welcome.
+
+## Security bugs
+
+Please report security bugs by filing a github issue, or by
+email to jbp@jbp.io if you want to disclose privately.  I'll then:
+
+- Prepare a fix and regression tests.
+- Backport the fix and make a patch release for most recent release.
+- Submit an advisory to [rustsec/advisory-db](https://github.com/RustSec/advisory-db).
+- Refer to the advisory on the main README.md and release notes.
+
+If you're *looking* for security bugs, this crate is set up for
+`cargo fuzz` but would benefit from more runtime, targets and corpora.
+
+## Testing
+
+- Features involving additions to the public API should have (at least)
+  API-level tests (see [`tests/api.rs`](tests/api.rs)).
+- Protocol additions should have some coverage -- consider enabling
+  corresponding tests in the bogo suite, or writing some adhoc tests.
+
+PRs which cause test failures or a significant coverage decrease
+are unlikely to be accepted.
+
+## Licensing
+
+Contributions are made under [rustls's licenses](LICENSE).