Add CONTRIBUTING.md

CONTRIBUTING.md

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+## Prerequisites
+
+Building Topola from source requires [git](https://git-scm.com/book/en/v2/Getting-Started-Installing-Git) and [cargo](https://doc.rust-lang.org/cargo/getting-started/installation.html) to be installed on your system.
+
+## Installation
+	
+Clone the [repository](https://codeberg.org/topola/topola):
+
+	git clone https://codeberg.org/topola/topola.git
+	
+Topola currently requires nightly Rust to build. Enable it with
+
+	rustup override set nightly
+		
+### egui port
+	
+After cloning, change your working directory to your clone of Topola's repository:
+
+    cd topola
+
+Then build the project with
+
+	cargo build --features egui --bin topola-egui	
+	
+Finally, run Topola by executing
+
+	cargo run --features egui --bin topola-egui
+
+#### Running Topola in a Web browser
+
+Topola can be built to run in a Web browser using [Trunk](https://trunkrs.dev/), which will be installed with the following command:
+
+    cargo binstall trunk
+
+To build and open Topola in your browser, run
+
+    trunk serve
+	
+### SDL2 demo
+
+For shorter build times you may use the SDL2 demo instead of the egui port:
+
+	cargo build --features sdl2 --bin topola-sdl2-demo	
+	cargo run --features sdl2 --bin topola-sdl2-demo
+
+The downside is that the SDL2 demo's user interface is highly incomplete.
+
+## Contributing
+
+Topola project accepts contributions using pull requests. For a step-by-step guide on how to use these, refer to Codeberg's [documentation](https://docs.codeberg.org/collaborating/pull-requests-and-git-flow/).