Modernize the README.

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

README

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-This is a 64 bit clean and bugfixed version of id Software's Quake II
-addon pack "Capture The Flag", developed by David 'Zoid' Kirsch. Hundred
-of type errors and strange constructs were fixed, thus this version
-should run much more stable than the old SDK version. While it may work
-with every Quake II client, the "Yamagi Quake II" client is highly
-recommended. 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. Copy release/game.so to the ctf/ subfolder of your Quake II
-   installation. The CTF gamedata is part of the 3.20 point release.
-3. Start the game with "./quake2 +set game ctf".
-
-Installation for OS X:
----------------------- 
-1. Copy game.dll from the zip-archive to ctf/.
-2. Start the game with "quake2 +set game ctf"
-
-If you want to compile 'ctf' 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. Copy the game.dll to the ctf\ subfolder of your Quake II
-   installation. The CTF gamedata is part of the 3.20 point release.
-2. Start the game with "quake2.exe +set game ctf".
-
-If you want to compile 'ctf' 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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+# Three Wave Capture The Flag for Yamagi Quake II
+
+Three Wave Capture The Flag for Yamagi Quake II is a bugfixed version of
+the official CTF addon for Quake II. It's based upon the Quake II source
+code release and and is licensed under GPL version 2:
+
+* [LICENSE](https://github.com/yquake2/ctf/blob/master/LICENSE)
+
+Several bugs were fixed. The addon 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 Three Wave Capture The Flag for Yamagi Quake II has
+community support for MacOS and most other unixoid operating systems,
+including NetBSD, OpenBSD and Solaris.
+
+
+## Development
+
+Three Wave Capture The Flag 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/ctf/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/ctf/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
+
+Three Wave Capture The Flag 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/ctf/actions)