Use rustfmt to enforce consistent formatting

This change adds a .rustfmt.toml that includes ALL supported settings,
12 of which we have overridden to attempt to cater to our own
proclivities.

rustfmt is checked in the rust-nightly CI job.

CONTRIBUTING.md

@@ -0,0 +1,95 @@
+# Contributing to _h2_ #
+
+:balloon: Thanks for your help improving the project!
+
+## Getting Help ##
+
+If you have a question about the h2 library or have encountered problems using it, you may
+[file an issue][issue] or ask ask a question on the [Tokio Gitter][gitter].
+
+## Submitting a Pull Request ##
+
+Do you have an improvement?
+
+1. Submit an [issue][issue] describing your proposed change.
+2. We will try to respond to your issue promptly.
+3. Fork this repo, develop and test your code changes. See the project's [README](README.md) for further information about working in this repository.
+4. Submit a pull request against this repo's `master` branch.
+6. Your branch may be merged once all configured checks pass, including:
+    - Code review has been completed.
+    - The branch has passed tests in CI.
+
+## Code Style ##
+
+[`rustfmt-nightly`][rustfmt] is used to enforce a consistent style (as configured by
+[`.rustfmt.toml`](./.rustfmt.toml)). Style is checked by CI, so be sure to apply rustfmt
+before committing your work.  For example:
+
+```
+$ rustup run nightly cargo install --force rustfmt-nightly
+    Updating registry `https://github.com/rust-lang/crates.io-index`
+  Installing rustfmt-nightly v0.2.5
+...
+$ rustup run nightly cargo fmt
+```
+
+## Committing ##
+
+We prefer squash or rebase commits so that all changes from a branch are
+committed to master as a single commit. All pull requests are squashed when
+merged, but rebasing prior to merge gives you better control over the commit
+message.
+
+### Commit messages ###
+
+Finalized commit messages should be in the following format:
+
+```
+Subject
+
+Problem
+
+Solution
+
+Validation
+```
+
+#### Subject ####
+
+- one line, <= 50 characters
+- describe what is done; not the result
+- use the active voice
+- capitalize first word and proper nouns
+- do not end in a period — this is a title/subject
+- reference the github issue by number
+
+##### Examples #####
+
+```
+bad: server disconnects should cause dst client disconnects.
+good: Propagate disconnects from source to destination
+```
+
+```
+bad: support tls servers
+good: Introduce support for server-side TLS (#347)
+```
+
+#### Problem ####
+
+Explain the context and why you're making that change.  What is the problem
+you're trying to solve? In some cases there is not a problem and this can be
+thought of as being the motivation for your change.
+
+#### Solution ####
+
+Describe the modifications you've made.
+
+#### Validation ####
+
+Describe the testing you've done to validate your change.  Performance-related
+changes should include before- and after- benchmark results.
+
+[issue]: https://github.com/carllerche/h2/issues/new
+[gitter]: https://gitter.im/tokio-rs/tokio
+[rustfmt]: https://github.com/rust-lang-nursery/rustfmt