Added readme.

README.md

+![Orekit logo](https://www.orekit.org/img/orekit-logo.png)
+
+# Orekit tutorials
+
+This project contains tutorials for the [Orekit](https://www.orekit.org) space dynamics library.
+
+## Download
+
+### Official releases
+
+[Official Orekit tutorial releases](https://gitlab.orekit.org/orekit/orekit-tutorials/releases)
+are available on our [Gitlab instance](https://gitlab.orekit.org/).
+
+### Development version
+
+To get the latest development version, please clone our official repository
+and checkout the `develop` branch:
+
+```bash
+git clone -b develop https://gitlab.orekit.org/orekit/orekit-tutorials.git
+```
+
+## Getting help
+
+The main communication channel is our [forum](https://forum.orekit.org/). You
+can report bugs and suggest new features in our
+[issues tracking system](https://gitlab.orekit.org/orekit/orekit-tutorials/issues). When
+reporting security issues check the "This issue is confidential" box.
+
+## Dependencies
+
+Orekit tutorials relies on the following
+[FOSS](https://en.wikipedia.org/wiki/Free_and_open-source_software) libraries,
+all released under business friendly FOSS licenses.
+
+### Compile-time/run-time dependencies
+
+* [Orekit](https://www.orekit.org/), a space flight dynamics library released under
+  the Apache License, version 2.0.
+
+* [Hipparchus](https://hipparchus.org/), a mathematics library released under
+  the Apache License, version 2.0.
+
+### Test-time dependencies
+
+* [JUnit 4](http://www.junit.org/), a widely used unit test framework released
+  under the Eclipse Public License, version 1.0.
+
+* [Mockito](https://site.mockito.org/), a mocking framework for unit tests,
+  released under MIT license.
+
+## License
+
+Orekit tutorials are licensed by [CS Systèmes d'Information](https://www.c-s.fr/) under
+the [Apache License, version 2.0](http://www.apache.org/licenses/LICENSE-2.0.html).
+A copy of this license is provided in the [LICENSE.txt](LICENSE.txt) file.