add README.md for GitHub mirror

Author: ralphlange

README.md

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+# The EPICS SNL Compiler and Sequencer
+
+This is a **mirror** of the *stable* (2.2) branch
+of the EPICS State Notation Language Compiler and Runtime Sequencer,
+converted to Git from an upstream-provided Darcs mirror
+at https://hub.darcs.net/bf/seq-branch-2-2
+
+## [Documentation](https://epics-modules.github.io/sequencer/)
+## [Releases](https://github.com/epics-modules/sequencer/tags)
+
+## Branches of this Mirror
+
+*   *main* - plain mirror of the upstream Darcs sources, plus this README file
+*   *ci* - branch that adds configuration for builds, tests and documentation
+           using GitHub Actions and Pages
+
+Upstream author/maintainer: [Benjamin Franksen](mailto:benjamin.franksen@helmholtz-berlin.de)