README.md in rake_docker-0.21.0 vs README.md in rake_docker-0.22.0.pre.pre.1

@@ -33,9 +33,32 @@
 release a new version, update the version number in `version.rb`, and then run
 `bundle exec rake release`, which will create a git tag for the version, push
 git commits and tags, and push the `.gem` file to
 [rubygems.org](https://rubygems.org).
 
+### Managing CircleCI keys
+
+To encrypt a GPG key for use by CircleCI:
+
+```bash
+openssl aes-256-cbc \
+  -e \
+  -md sha1 \
+  -in ./config/secrets/ci/gpg.private \
+  -out ./.circleci/gpg.private.enc \
+  -k "<passphrase>"
+```
+
+To check decryption is working correctly:
+
+```bash
+openssl aes-256-cbc \
+  -d \
+  -md sha1 \
+  -in ./.circleci/gpg.private.enc \
+  -k "<passphrase>"
+```
+
 ## Contributing
 
 Bug reports and pull requests are welcome on GitHub at 
 https://github.com/infrablocks/rake_docker. This project is intended to be a 
 safe, welcoming space for collaboration, and contributors are expected to