Modernize the README.

Convert to markdown and follow the same style as the yquake2 main
repository. Use it's resources were appropriate.

README

@@ -1,41 +0,0 @@
-This is a bugfixed version of id Software's Quake II missionpack
-"Ground Zero", developed by Rogue Software. Hundreds of bugs were
-fixed, this version should run much more stable than the old
-SDK version. While compatible with any Quake II client that uses
-the original unaltered mod API, the "Yamagi Quake II Client" is
-highly recommended to play the addon. For more information visit
-http://www.yamagi.org/quake2. 
-
-Installation for FreeBSD, Linux and OpenBSD:
---------------------------------------------
-1. Type "make" or "gmake" to compile the game.so.
-2. Create a subdirectory rogue/ in your quake2 directory.
-3. Copy pak0.pak and videos/ from the "Ground Zero" CD to
-   the newly created directory rogue/.
-4. Copy release/game.so to rogue/.
-5. Start the game with "./quake2 +set game rogue"
-
-Installation for OS X:
-----------------------
-1. Create a subdirectory rogue/ in your quake2 directory.
-2. Copy pak0.pak and videos/ from the "Ground Zero" CD to
-   the newly created directory rogue/.
-3. Copy game.dynlib from the zip-archive to rogue/.
-4. Start the game with "quake2 +set game rogue"
-
-If you want to compile 'rogue' for OS X from source, please take a
-look at the "Installation" section of the README of the Yamagi Quake II
-client. In the same file the integration into an app-bundle is
-explained.
- 
-Installation for Windows:
--------------------------
-1. Create a subdirectory rogue\ in your quake2 directory.
-2. Copy pak0.pak and videos\ from the "Ground Zero" CD to
-   the newly created directory rogue\.
-3. Copy game.dll from the zip-archive to rogue/.
-4. Start the game with "quake2.exe +set game rogue"
-
-If you want to compile 'rogue' for Windows from source, please take a
-look at the "Installation" section of the README of the Yamagi Quake II
-client. There's descripted how to setup the build environment. 

README.md

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+# Ground Zero for Yamagi Quake II
+
+Ground Zero for Yamagi Quake II is a bugfixed version of the second
+official missionpack released for Quake II. It's based upon the Quake
+II SDK source code and licensed under GPL version 2:
+
+* [LICENSE](https://github.com/yquake2/rogue/blob/master/LICENSE)
+
+Hundreds of bugs were fixed and some convenience features added. The
+missionpack is intended to be used with Yamagi Quake II, but it's also
+fully backward compatible with the last Quake II pointrelease 3.20 and
+may work with other source ports.
+
+Officially supported operating systems are:
+
+* FreeBSD
+* Linux
+* Windows
+
+Beside theses Ground Zero for Yamagi Quake II has community support
+for MacOS and most other unixoid operating systems, including NetBSD,
+OpenBSD and Solaris.
+
+
+## Development
+
+Ground Zero for Yamagi Quake II is a community driven project and
+lives from community involvement. Please report bugs in our issue
+tracker:
+
+* [Issue Tracker](https://github.com/yquake2/rogue/issues)
+
+We are always open to code contributions, no matter if they are small
+bugfixes or bigger features. However, Yamagi Quake II is a conservative
+project with big focus on stability and backward compatibility. We don't
+accept breaking changes. When in doubt please open an issue and ask if a
+contribution in welcome before putting too much work into it. Open a
+pull request to submit code:
+
+* [Pull Requests](https://github.com/yquake2/rogue/pulls)
+
+Also have a look at our contributors guide:
+
+* [Contributors Guide](https://github.com/yquake2/yquake2/blob/master/doc/080_contributing.md)
+
+
+## Documentation
+
+Yamagi Quake II has rather extensive documentation covering all relevant
+areas from installation and configuration to package building. Have a
+look at the documentation index:
+
+* [Documentation Index](https://github.com/yquake2/yquake2/blob/master/doc/010_index.md)
+
+
+## Releases
+
+Ground Zero for Yamagi Quake II releases at an irregular schedule. The
+official releases with source code tarballs and prebuild Windows
+binaries can be found at the homepage:
+
+* [Homepage](https://www.yamagi.org/quake2/)
+
+Our CI builds **unsupported** Linux, MacOS and Windows binaries at every
+commit. The artifacts can be found here:
+
+* [Github Actions](https://github.com/yquake2/rogue/actions)