Book: Draft for exit code chapter

Resolves rust-cli#39

src/in-depth/exit-code.md

@@ -1 +1,55 @@
 # Exit codes
+
+A program doesn't always success.
+And when an error occurs,
+you should make sure to use the correct ways to emit the necessary information.
+In addition to
+[telling the user about errors](human-communication.md),
+on most systems,
+when a process exits,
+it also emits an exit code
+(an integer between 0 and 255).
+You should try to emit the correct code
+for your program's state.
+For example,
+in the ideal case when your program succeeded,
+it should exit with `0`.
+
+When an error ocurred, it gets a bit more complicated, though.
+In the wild,
+a lot of tools exit with `1` when a general failure ocurred.
+Currently, Rust set and exit code of `101` when the process panicked.
+Beyond that, many people have done many things in their programs.
+
+So, what to do?
+The BSD ecosystem has collected a common definition for their exit codes
+in a system-provided header file called [`sysexits.h`]
+The Rust library [`exitcode`] provides these same codes
+ready to be used in your application.
+Please see it's API documentation for the possible values to use.
+
+One way to use it is like this:
+
+```rust
+fn main() {
+    // ...actual work...
+    match result {
+        Ok(_) => {
+            println!("Done!");
+            std::process::exit(exitcode::OK);
+        }
+        Err(CustomError::CantReadConfig(e)) => {
+            eprintln!("Error: {}", e);
+            std::process::exit(exitcode::CONFIG);
+        }
+        Err(e) => {
+            eprintln!("Error: {}", e);
+            std::process::exit(exitcode::DATAERR);
+        }
+    }
+}
+```
+
+
+[`exitcode`]: https://crates.io/crates/exitcode
+[`sysexits.h`]: https://www.freebsd.org/cgi/man.cgi?query=sysexits&apropos=0&sektion=0&manpath=FreeBSD+11.2-stable&arch=default&format=html