Add sncast guidelines to CONTRIBUTING.md (#2843)

<!-- Reference any GitHub issues resolved by this PR -->

Closes #2604

## Introduced changes

Add `sncast` contribution guidelines

## Checklist

<!-- Make sure all of these are complete -->

- [x] Linked relevant issue
- [x] Updated relevant documentation
- [x] Added relevant tests
- [x] Performed self-review of the code
- [x] Added changes to `CHANGELOG.md`

Author: franciszekjob

CONTRIBUTING.md

@@ -40,3 +40,15 @@ Creating very large pull requests may significantly increase review time or even
 
 At this time, we will not be accepting contributions that only fix spelling or grammar errors in documentation, code or
 elsewhere.
+
+### `sncast` Guidelines
+
+#### Command Outputs
+
+Please follow these rules when creating outputs for `sncast`:
+
+- Use an imperative tone
+- Keep your message concise and to the point
+- When displaying config, use `key: value` format
+- If the executed command has a natural successor-command, display it as hint in the output. For example, the output of `declare` command should include a hint to use `deploy` command next.
+<!-- TODO(#2859): Add bullet point about colors used for text when displaying fees, addresses and hashes -->