readme: add instructions on how to build project

includes building using:
* nix flake functionality ('nix develop'+the go tool provided by nix)
* just the go tool

README.md

@@ -5,5 +5,30 @@
 ### disclaimer
 WIP :construction:
 
+### ad nix flake ❄
+this project is provided as a **nix flake** for both convenience and
+reproducibility.  
+and also because I wanted to play with `nix` and its flakes just a wee bit.
+
+> *note on the nix-related steps*: make sure you have
+> [`nix`](https://nixos.org/learn.html) installed and optionally the flake
+> functionality enabled, the Go toolchain and a minimal set of Go development
+> goodies will get pulled in automatically.
+
+that means you can get started:
+### developing using nix (flakes enabled)
+run `nix develop` in the project folder *first*, then `go {run,build,fmt,...}`
+about your day.
+
+### developing without nix
+> *note*: you only need to have the Go toolchain and [Fyne
+> dependencies](https://developer.fyne.io/started/#prerequisites) installed for
+> this.
+
+if you don't want to use the flake provided or don't have `nix` installed, you
+can use the standard `go` tool:
+run `go {run,build,fmt,...}` in the project folder.
+
+
 ### COPYING
 Licensed under GPL-3.0-or-later (see [LICENSE](LICENSE) for details).