Name Chris Miller as the backup maintainer

GOVERNANCE.md

@@ -2,8 +2,9 @@
 
 Intersection Lab is a one-maintainer project. Jim Schreckengast
 ([schreck61](https://github.com/schreck61)) started it, maintains it and makes the final
-decisions about it. This document describes how those decisions are made and who does
-what.
+decisions about it. Chris Miller ([csm0513](https://github.com/csm0513)) is the backup
+maintainer, ready to keep the project going if Jim can't (see [Continuity](#continuity)).
+This document describes how decisions are made and who does what.
 
 ## How decisions are made
 
@@ -32,9 +33,33 @@ what.
 | Role | Who | Responsibilities |
 |---|---|---|
 | Maintainer | Jim Schreckengast ([schreck61](https://github.com/schreck61)) | Sets the project's direction; triages issues; reviews and merges pull requests; keeps CI, dependencies and documentation up to date; makes releases and publishes them on GitHub and crates.io; manages the code-signing and publishing settings; responds to vulnerability reports as [SECURITY.md](SECURITY.md) describes; enforces the [Code of Conduct](CODE_OF_CONDUCT.md) |
+| Backup maintainer | Chris Miller ([csm0513](https://github.com/csm0513)) | Keeps the project going if the maintainer can't, as [Continuity](#continuity) describes: answers and closes issues, merges pull requests whose required checks pass, makes releases, and settles the project's long-term future; has no duties otherwise |
 | Contributor | Anyone who opens an issue or pull request | Follows [CONTRIBUTING.md](CONTRIBUTING.md), signs off every commit under the [Developer Certificate of Origin](https://developercertificate.org/), and follows the Code of Conduct |
 | Security reporter | Anyone who finds a vulnerability | Reports it privately as [SECURITY.md](SECURITY.md) describes, not in a public issue |
 
+## Continuity
+
+If the maintainer dies, is incapacitated or can no longer maintain the project, the
+backup maintainer keeps it going. Within a week of learning of it, they answer and close
+issues, merge pull requests whose required checks pass, and make releases. They then
+settle the project's long-term future: carrying it on, handing it to a new maintainer,
+or archiving it.
+
+The backup maintainer already has the access this takes:
+
+* write access to this repository, which covers issues, pull requests and the version
+  tags that build and publish releases;
+* co-ownership of the [crate on crates.io](https://crates.io/crates/intersection-lab);
+* the maintainer's GitHub *successor* designation, under which GitHub can transfer this
+  repository to the backup maintainer if the maintainer dies.
+
+No passwords or keys change hands. Every release is built, signed and published by
+GitHub Actions rather than on anyone's own computer, and if the code-signing accounts
+lapse, the release workflow can be switched to unsigned packages (see
+[packaging/README.md](packaging/README.md#making-a-release)). The backup maintainer has
+written instructions for each step, and the maintainer and the backup maintainer check
+this access once a year.
+
 ## Changing this document
 
 If the project gains more maintainers, this document will be updated to say how they