Import versioning guidance from dev doc

This is part of work to rewrite the "Pact Broker" dev doc [^1] so
that it's focussed on writing tests, not details about this repo.

[^1]: https://github.com/alphagov/govuk-developer-docs/blame/f8c7344ae79f333cd927cfb9ba30be2d2ee58fa4/source/manual/pact-broker.html.md#L54

README.md

@@ -43,6 +43,7 @@ $ bundle exec unicorn
 
 - [Infrastructure](docs/infrastructure.md)
 - [Deployment](docs/deployment.md)
+- [Versioning](docs/versioning.md)
 
 [pact-broker-gem]: https://github.com/bethesque/pact_broker
 [government-paas]: https://docs.cloud.service.gov.uk/

docs/versioning.md

@@ -0,0 +1,16 @@
+# Versioning
+
+Out of the box, the Pact Broker allows uploading of pact files with semver
+style versions (eg 2.0.1). For our usage, we wanted to be able to upload pact
+files from various branches in addition the released versions so that our
+branch builds of consumers can verify their pactfiles with the producers.
+
+Pact Broker allows us to [implement our own versioning
+scheme](https://github.com/bethesque/pact_broker/wiki/Configuration#version-parser)
+by providing a custom version parser.  We've [used
+this](https://github.com/alphagov/govuk-pact-broker/blob/master/config.ru#L23-L50)
+to extend the versioning scheme to allow branch builds to be uploaded as well.
+In addition to numeric versions, our scheme allows for "master", and
+"branch-foo" versions to be uploaded. These will always be ordered after any
+numeric versions, so the 'latest' pactfile from the pact brokers POV will
+always be the highest numeric version.