docs(commands): Add info about clap Derive API (#2066)

Add information to contributing docs about using Derive API for future commands.

CONTRIBUTING.md

@@ -1,3 +1,12 @@
+# Adding new commands
+For new commands, it is recommended to use clap's [Derive API](https://docs.rs/clap/latest/clap/_derive/index.html). 
+In contrast to the [Builder API](https://docs.rs/clap/latest/clap/_tutorial/index.html), the Derive API makes it:
+- Easier to read, write, and modify commands and arguments.
+- Easier to keep argument declaration and reading in sync.
+- Easier to reuse shared arguments.
+
+An existing example of how to use the Derive API is the `send-metric` command.
+
 # Integration Tests
 
 Integration tests are written using `trycmd` crate. Consult the docs in case you need to understand how it works https://docs.rs/trycmd/latest/trycmd/.