add release documentation to CONTRIBUTING.md (#1171)

## Motivation

Currently, the process for releasing a new version of a Tokio crate is
somewhat complex, and is not well-documented. To make it easier for
contributors to release minor versions more frequently, there should be
documentation describing this process.

## Solution

This branch adds a section to `CONTRIBUTING.md` describing how to
release a new version of a Tokio crate. The steps are based on those
described by @carllerche in an offline conversation.

I've also added a quick shell script to actually publish new crate 
versions. This should make it harder to make mistakes when 
publishing.

Signed-off-by: Eliza Weisman <[email protected]>

CONTRIBUTING.md

@@ -381,3 +381,63 @@ _Adapted from the [Node.js contributing guide][node]_.
 [node]: https://github.com/nodejs/node/blob/master/CONTRIBUTING.md
 [hiding-a-comment]: https://help.github.com/articles/managing-disruptive-comments/#hiding-a-comment
 [documentation test]: https://doc.rust-lang.org/rustdoc/documentation-tests.html
+
+## Releasing
+
+Since the Tokio project consists of a number of crates, many of which depend on
+each other, releasing new versions to crates.io can involve some complexities.
+When releasing a new version of a crate, follow these steps:
+
+1. **Ensure that the release crate has no path dependencies.** When the HEAD
+   version of a Tokio crate requires unreleased changes in another Tokio crate,
+   the crates.io dependency on the second crate will be replaced with a path
+   dependency. Crates with path dependencies cannot be published, so before
+   publishing the dependent crate, any path dependencies must also be published.
+   This should be done through a form of depth-first tree traversal:
+
+   1. Starting with the first path dependency in the crate to be released,
+      inspect the `Cargo.toml` for the dependency. If the dependency has any
+      path dependencies of its own, repeat this step with the first such
+      dependency.
+   2. Begin the release process for the path dependency.
+   3. Once the path dependency has been published to crates.io, update the
+      dependent crate to depend on the crates.io version.
+   4. When all path dependencies have been published, the dependent crate may
+      be published.
+
+   To verify that a crate is ready to publish, run:
+
+   ```bash
+   bin/publish --dry-run <CRATE NAME> <CRATE VERSION>
+   ```
+
+2. **Update Cargo metadata.** After releasing any path dependencies, update the
+   `version` field in `Cargo.toml` to the new version, and the `documentation`
+   field to the docs.rs URL of the new version.
+3. **Update other documentation links.** Update the `#![doc(html_root_url)]`
+   attribute in the crate's `lib.rs` and the "Documentation" link in the crate's
+   `README.md` to point to the docs.rs URL of the new version.
+4. **Update the changelog for the crate.** Each crate in the Tokio repository
+   has its own `CHANGELOG.md` in that crate's subdirectory. Any changes to that
+   crate since the last release should be added to the changelog. Change
+   descriptions may be taken from the Git history, but should be edited to
+   ensure a consistent format, based on [Keep A Changelog][keep-a-changelog].
+   Other entries in that crate's changelog may also be used for reference.
+5. **Perform a final audit for breaking changes.** Compare the HEAD version of
+   crate with the Git tag for the most recent release version. If there are any
+   breaking API changes, determine if those changes can be made without breaking
+   existing APIs. If so, resolve those issues. Otherwise, if it is necessary to
+   make a breaking release, update the version numbers to reflect this.
+6. **Open a pull request with your changes.** Once that pull request has been
+   approved by a maintainer and the pull request has been merged, continue to
+   the next step.
+7. **Release the crate.** Run the following command:
+
+   ```bash
+   bin/publish <NAME OF CRATE> <VERSION>
+   ```
+
+   Your editor and prompt you to edit a message for the tag. Copy the changelog
+   entry for that release version into your editor and close the window.
+
+[keep-a-changelog]: https://github.com/olivierlacan/keep-a-changelog/blob/master/CHANGELOG.md