contributing: add some introductory and welcoming text

CONTRIBUTING.md

@@ -1,8 +1,50 @@
-## Prerequisites
+# Contributing to Topola
 
-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.
+Literally anyone can contribute to Topola, including you.
 
-## Installation
+Contributions can be of any kind: documentation, organization,
+tutorials, blog posts, bug reports, issues, feature requests, feature
+implementations, pull requests, helping to manage issues, etc.. Many of
+these tasks do not require specialized programming knowledge, or writing
+software at all.
+
+## Chat
+
+You are encouraged to join our [Matrix
+chatroom](https://matrix.to/#/%23topola:tchncs.de) or [IRC
+channel](https://webchat.oftc.net/?channels=#topola) to talk with us
+before you contribute. Both chatrooms are bridged, so it does not matter
+which one you join.
+
+## Reporting issues
+
+If you believe you have found a defect in Topola or its documentation,
+please report it on our [issue tracker](https://codeberg.org/topola/topola/issues).
+
+Under normal operation, crashes and panics are always considered
+reportable bugs.
+
+## Code contribution
+
+We welcome code contributions from anyone regardless of skill or
+experience level. We're friendly to newcomers. We will help you with
+your contribution if there are any problems.
+
+Topola 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/).
+
+Before you submit a pull request, make sure Topola actually builds with
+your changes. Follow the build instructions from the next section.
+
+### Building
+
+#### 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.
 
 Clone the [repository](https://codeberg.org/topola/topola):
 
@@ -12,9 +54,9 @@ Topola currently requires nightly Rust to build. Enable it with
 
     rustup override set nightly
 
-### egui port
+#### Egui port
 
-After cloning, change your working directory to your clone of Topola's repository:
+Change your working directory to your clone of Topola's repository:
 
     cd topola
 
@@ -26,9 +68,11 @@ Finally, run Topola by executing
 
     cargo run --features egui --bin topola-egui
 
-#### Running Topola in a Web browser
+##### 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:
+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
 
@@ -36,15 +80,12 @@ To build and open Topola in your browser, run
 
     trunk serve
 
-### SDL2 demo
+#### SDL2 demo
 
-For shorter build times you may use the SDL2 demo instead of the egui port:
+Optionally, for shorter build times you may build 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/).