Add a code review policy that clarifies:

1. Who "owns" libtock-rs code (who reviews it)
2. Who is responsible for merging PRs.
3. When a PR may be merged.

This is based on Tock's policy that is currently at https://github.com/tock/tock/blob/master/doc/CodeReview.md

doc/CodeReview.md

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+Code Review
+===========
+
+## Code Review Practices
+
+PR to `libtock-rs` can be divided into two categories:
+
+1. **Upkeep pull requests** are minor changes to existing functionality.
+   Examples include bug fixes (that do not significantly affect APIs) and
+   documentation that describes an existing implementation.
+1. **Significant pull requests** are pull requests that are too substantial to
+   be considered upkeep pull requests. Significant pull requests may include new
+   functionality, API changes, significant refactoring, new tooling, and other
+   changes.
+
+The owners of `libtock-rs` (listed [below](#owners)) determine whether a PR is
+an upkeep PR or a significant PR. PRs should be merged by the `libtock-rs`
+owners rather than the PR's author. PRs authored by `libtock-rs` owners should
+be merged by a reviewer rather than their author.
+
+A PR may only be merged when all of the following are true:
+
+1. At least one `libtock-rs` owner (who is not the PR author) has approved the PR.
+1. All outstanding review discussions have been resolved.
+1. If the pull request is significant, a 7 day waiting period has passed since
+   the PR was opened.
+
+## Owners
+
+The owners of `libtock-rs` are:
+
+* The [Tock Core Working
+  Group](https://github.com/tock/tock/tree/master/doc/wg/core#members).
+* Alistair Francis, [alistair23](https://github.com/alistair23), Western Digital
+* [torfmaster](https://github.com/torfmaster)
+* [Woyten](https://github.com/Woyten)