Add a readme file

+# Rust playground
+
+Things I have recently done while learning the Rust programming language.
+
+## Synopsis
+
+Change in one of the toplevel subdirectories and try `cargo build` or
+`cargo run`. Maybe not everything is working oob. Feel free to fix whatever
+you want.
+
+## Description
+
+Various small examples I have tried while learning rust. The biggest and
+currently most active project is **fractional** which started as an
+implamentation of a rational number data type and then switched to a 3D
+math playground visualizing using **XCB** (in future it might also use
+a **HTML5 Canvas** for drawing as WebAssembly application.
+Using fractions with 3D math has several drawbacks:
+
+1. A huge part of 3D math is non rational, like sin, cos, tan and sqrt.
+2. The numerator and denominator tend to become very huge while nearing to non
+   rational numbers and reduction is difficult and time consuming.
+3. Because of 2 it is way slower than the floating point calculation (at least
+   with a decent coprocessor).
+
+Anyway, implementing the vector math stuff for both fractions and floating
+point was a nice playground for generics and traits. In future I might add
+another data type which implements the math as done by David Braben for the
+elite computer game.
+
+## Requirements
+
+### Always
+
+- A recent version of the Rust programming language as well as tooling.
+  Currently I use Rust 1.39.0.
+
+### For fractional
+
+- A running X Server with **XCB** and **X11-SHM** extentions
+
+## Dependencies
+
+...
+
+## Contributing
+
+Feel free to make pull requests as you like, no guarantee that the will be
+added.
+
+## License
+
+Copyright © 2020 Georg Hopp
+
+This program is free software: you can redistribute it and/or modify
+it under the terms of the GNU General Public License as published by
+the Free Software Foundation, either version 3 of the License, or
+(at your option) any later version.
+
+This program is distributed in the hope that it will be useful,
+but WITHOUT ANY WARRANTY; without even the implied warranty of
+MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+GNU General Public License for more details.
+
+You should have received a copy of the GNU General Public License
+along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+## Author
+
+Georg Hopp <georg@steffers.org>