Restore top level README (input-output-hk#2655)

Author: erikd

README.md

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+# Cardano SL
+
+[![Build status](https://badge.buildkite.com/9c3141d21214ff3ea95d0a38a0e1dab59b206159d2841dee44.svg?branch=master)](https://buildkite.com/input-output-hk/cardano-sl)
+[![Windows build status](https://ci.appveyor.com/api/projects/status/github/input-output-hk/cardano-sl?branch=master&svg=true)](https://ci.appveyor.com/project/input-output/cardano-sl)
+[![Release](https://img.shields.io/github/release/input-output-hk/cardano-sl.svg)](https://github.com/input-output-hk/cardano-sl/releases)
+
+## What is Cardano SL?
+
+Cardano SL (or Cardano Settlement Layer) is a cryptographic currency designed
+and developed by [IOHK](https://iohk.io/team). Please read [Cardano SL Introduction](https://cardanodocs.com/introduction/)
+for more information.
+
+## Supported Platforms
+
+Please see [Cardano SL Installation](https://cardanodocs.com/installation/) for more
+information.
+
+## Building Cardano SL from Source Code
+
+Please see [this guide](https://cardanodocs.com/for-contributors/building-from-source/) for
+more information.
+
+## For Contributors
+
+Please see [CONTRIBUTING.md](https://github.com/input-output-hk/cardano-sl/blob/develop/CONTRIBUTING.md)
+for more information.
+
+## License
+
+Please see [LICENSE](https://github.com/input-output-hk/cardano-sl/blob/master/LICENSE) for
+more information.