Update README with state of project, screenshots

README

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-Welcome to XEphem, an interactive astronomy program for all UNIX platforms.
-
-Start with the INSTALL file.

README.md

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+
+# Welcome to XEphem!
+
+XEphem is an interactive astronomy program for all UNIX platforms.
+
+![Sky view](/GUI/xephem/help/png/sky-view.png?raw=true)
+
+More screenshots are shown below.
+
+* Documentation lives at <https://www.clearskyinstitute.com/xephem/>
+  where Elwood Downey —
+  XEphem’s original author and its maintainer since the 1990s —
+  offers a manual, data files, and user contributed scripts.
+
+* Elwood Downey has generously released XEphem under the MIT License!
+  An “XEphem” organization has been created on GitHub
+  to host XEphem’s [source code](https://github.com/XEphem/XEphem).
+
+* The XEphem community hopes to modernize the code
+  to make it easier to compile, package, and run on modern systems.
+  In the meantime,
+  XEphem compiles most readily on older Linux platforms
+  like Ubuntu 14.04.
+
+* If you want to get started immediately,
+  a Docker image
+  that successfully compiles and runs XEphem atop Ubuntu 14.04
+  is available in the “docker” branch on GitHub.
+  It’s documented
+  [in this README file](https://github.com/XEphem/XEphem/blob/docker/container/README.md).
+
+* Otherwise,
+  if you want to tackle XEphem development on another platform,
+  start with the
+  [INSTALL](https://github.com/XEphem/XEphem/blob/main/INSTALL)
+  file in the `main` branch of the repository.
+
+* Direct questions to the main repository’s
+  [GitHub Discussions](https://github.com/XEphem/XEphem/discussions)
+  by clicking “New discussion” and writing up your question.
+
+A few more screenshots:
+
+![Sun view](/GUI/xephem/help/png/sun-view.png?raw=true)
+
+![Solar system view](/GUI/xephem/help/png/solsys.png?raw=true)
+
+![Saturn and its moons](/GUI/xephem/help/png/saturn.png?raw=true)
+
+![Binary star view](/GUI/xephem/help/png/sky-binary.png?raw=true)
+
+![Moon view](/GUI/xephem/help/png/moon.png?raw=true)
+
+![Earth view](/GUI/xephem/help/png/earth.png?raw=true)