docs: add documentation for developers

CONTRIBUTING.md

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+# Developing hcloud-cli
+
+## Generated files
+This repository contains generated files, mainly for testing purposes. These files are generated by running
+```sh
+go generate ./...
+```
+in the root directory of this repository. Make sure to keep generated files up-to-date
+when making changes to the code.
+
+## Unit tests
+Unit tests are located in the `internal` directory. Run them with 
+```sh
+go test ./...
+```
+
+## Build
+To build the binary, run 
+```sh
+go build -o hcloud-cli ./cmd/hcloud
+```
+
+To include version information in the resulting binary and build for all targets, use GoReleaser: 
+```sh
+goreleaser --snapshot --skip-publish --rm-dist
+```