docs: add documentation

README.md

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+# Nixtest
+
+[![built with nix](https://img.shields.io/static/v1?logo=nixos&logoColor=white&label=&message=Built%20with%20Nix&color=41439a)](https://builtwithnix.org)
+[![pipeline status](https://gitlab.com/TECHNOFAB/nixtest/badges/main/pipeline.svg)](https://gitlab.com/TECHNOFAB/nixtest/-/commits/main)
+![License: MIT](https://img.shields.io/gitlab/license/technofab/nix-gitlab-ci)
+[![Latest Release](https://gitlab.com/TECHNOFAB/nixtest/-/badges/release.svg)](https://gitlab.com/TECHNOFAB/nixtest/-/releases)
+[![Support me](https://img.shields.io/badge/Support-me-green)](https://tec.tf/#support)
+[![Docs](https://img.shields.io/badge/Read-Docs-green)](https://nixtest.projects.tf)
+
+Flexible test runner for testing Nix code, written in Go.
+
+## Features
+
+- Snapshot, Unit (equal checks) and Script-Tests (unit tests with assertions you could say)
+- Supports testing against raw Nix code or derivation output
+- Simple and easy to read summary of test results
+- Junit report support (eg. for displaying the results in GitLab etc.)
+
+## Usage
+
+See the [docs](https://nixtest.projects.tf/usage).