Add README.md.

README.md

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+Jenkins script (Jenkinsfile) for  official Devuan package building jobs
+=======================================================================
+
+The Jenkinsfile in this repository is used by Devuan's [jenkins
+instance](https://ci.devuan.org) to build and deploy the official Devuan
+packages.
+
+Note that [jenkins](https://ci.devuan.org) sources the Jenkinsfile directly for
+each build so any changes here will be used for any subsequent builds that are
+triggered.
+
+The significant branches are:-
+
+## Deployment branch
+
+Used by the [production
+builder](https://jenkins.devuan.dev/job/devuan-package-builder/) to build
+official packages.
+
+## Testing branch
+
+Used in the [development and testing
+builder](https://ci.devuan.org/job/test-pipeline) which builds in the same way
+but does not notify [#devuan-ci](irc://libera.chat/devuan-ci) or deploy
+artifacts to DAK.
+
+## Master branch
+
+The default branch.