move out developer guide out of reference

Author: Mic92

CONTRIBUTING.md

@@ -0,0 +1,23 @@
+To run `nixos-anywhere` from the repo:
+
+```console
+nix run . -- --help
+```
+
+To format the code:
+
+```console
+nix fmt
+```
+
+To run all tests:
+
+```console
+nix flake check -vL
+```
+
+To run an individual test:
+
+```
+nix build .#checks.x86_64-linux.from-nixos -vL
+```

docs/reference.md

@@ -49,20 +49,6 @@ Options:
   build the closure on the remote machine instead of locally and copy-closuring it
 ```
 
-## Developer guide
-
-To run `nixos-anywhere` from the repo:
-
-```shell
-nix run . -- --help
-```
-
-To format the code
-
-```shell
-nix fmt
-```
-
 ## Explanation of known error messages
 
 TODO: List actual error messages and meanings. Include: