docs: CONTRIBUTING.md (#439)

CONTRIBUTING.md

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+## Contributing to `topgrade`
+
+Thank you for your interest in contributing to `topgrade`! We welcome and encourage
+contributions of all kinds, such as:
+
+1. Issue reports or feature requests
+2. Documentation improvements
+3. Code (PR or PR Review)
+
+## Adding a new `step`
+
+In `topgrade`'s term, package manager is called `step`. To add a new `step` to
+`topgrade`: 
+
+1. Add a new variant to 
+   [`enum Step`](https://github.com/topgrade-rs/topgrade/blob/cb7adc8ced8a77addf2cb051d18bba9f202ab866/src/config.rs#L100)
+
+   ```rust
+   pub enum Step {
+       // Existed steps
+       // ...
+
+       // Your new step here!
+       // You may want it to be sorted alphabetically becauses that looks great:)
+       Xxx,
+   }
+   ```
+
+2. Implement the update function
+
+   You need to find the appropriate location where this update function goes, it should be
+   a file under [`src/steps`](https://github.com/topgrade-rs/topgrade/tree/master/src/steps),
+   the file names are self-explanatory, for example, `step`s related to `zsh` are
+   placed in [`steps/zsh.rs`](https://github.com/topgrade-rs/topgrade/blob/master/src/steps/zsh.rs).
+
+   Then you implement the update function, and put it in the file where it belongs.
+
+   ```rust
+   pub fn run_xxx(ctx: &ExecutionContext) -> Result<()> {
+       // Check if this step is installed, if not, then this update will be skipped.
+       let xxx = require("xxx")?;
+
+       // Print the separator
+       print_separator("xxx");
+
+       // Invoke the new step to get things updated!
+       ctx.run_type()
+          .execute("xxx")
+          .arg(/* args required by this step */)
+          .status_checked()
+   }
+   ```
+
+   Such a update function would be conventionally named `run_xxx()`, where `xxx`
+   is the name of the new step, and it should take a argument of type 
+   `&ExecutionContext`, this is adequate for most cases unless some extra stuff is
+   needed (e.g., [you need to cleanup after update](https://github.com/topgrade-rs/topgrade/blob/cb7adc8ced8a77addf2cb051d18bba9f202ab866/src/steps/os/windows.rs#L56)).
+
+   Update function would usually do 3 things:
+   1. Check if the step is installed
+   2. Output the Separator
+   3. Invoke the step
+
+   Still, this is sufficient for most tools, but you may need some extra stuff
+   with complicated `step`.
+
+3. Finally, invoke that update function in `main.rs`
+
+   ```rust
+   runner.execute(Step::Xxx, "xxx", || ItsModule::run_xxx(&ctx))?;
+   ```
+
+   We use [conditional compilation](https://doc.rust-lang.org/reference/conditional-compilation.html)
+   to separate the steps, for example, for steps that are Linux-only, it goes
+   like this:
+
+   ```
+   #[cfg(target_os = "linux")]
+   {
+       // Xxx is Linux-only
+       runner.execute(Step::Xxx, "xxx", || ItsModule::run_xxx(&ctx))?;
+   }
+   ```
+
+   Congrats, you just added a new `step`:)
+
+## Before you submit your PR
+
+Make sure your patch passes the following tests on your host:
+
+```shell
+$ cargo build
+$ cargo fmt
+$ cargo clippy
+$ cargo test
+```
+
+Don't worry about other platforms, we have most of them covered in our CI.
+
+## Some tips
+
+1. Locale
+
+   Some `step` respects locale, which means their output can be in language other
+   then English, we should not do check on it.
+
+   For example, one may want to check if a tool works by doing this:
+
+   ```rust
+   let output = Command::new("xxx").arg("--help").output().unwrap();
+   let stdout = from_utf8(output.stdout).expect("Assume it is UTF-8 encoded");
+
+   if stdout.contains("help") {
+       // xxx works
+   }
+   ```
+
+   If `xxx` respects locale, then the above code should work on English system,
+   on a system that does not use English, e.g., it uses Chinese, that `"help"` may be 
+   translated to `"帮助"`, and the above code won't work.