The CLI tool could really need some love here with:

• Some simple parameter check
• Some help/example output on wrong usage + error cases
• An explicit example for encoding and decoding in the README (e.g. from the tests: rlp encode "[ 5 ]")

The algorithm to decode an be found here on ethereum/wiki/RLP, they also have an implementation example in js.

Suggestion for the function signature:
func decode(_ data: Data) throws -> Data