optimize codes, fix flake, and add license & docs

docs/develop.md

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+# How to develop
+
+Thanks to the `nix`, we can easily develop the project in a reproducible environment. The following steps will guide you through the process.
+
+## nix environment
+> TODO
+
+## shell interactions
+
+There are several integrated nix commands that can help you develop the project.
+
+- `nix develop`
+    enter the default development shell
+- `nix build`
+    build the project, the result will be at `result/bin/faas-rs`
+- `nix flake check`
+    do the CI checks, including formatting, linting...
+
+Besides, you can also use the `cargo` command to interact with the project in the shell. Nix will automatically manage the toolchain and library dependencies for you.
+
+> [!NOTE]
+> The project uses `cargo-hakari`, to optimize the build time.
+
+If you don't know what it is, after new dependencies are added, you should run `cargo hakari generate` to update the dependencies.