For the most part, try to follow the Rust Style Guide.
If your contribution adds a new command, give it its own file in src/commands. If your contribution adds a significant new feature, give it its own module (like the IRC bridge in src/irc_bridge and the SMTP api in src/smtp). Ensure that your code is clean and readable so that others can read and add to it.
If you need to add new dependencies, try using feature flags to only include the features you need (thus minimizing the number of new dependencies that they in turn bring in)
Test your new features locally by creating a Discord bot and following the instructions in README.md for setting the token in an environment variable. A config.json containing just {} is valid and will disable most bot features.
Once your new features are complete and fully tested, create a pull request against the main repo.