Add CHANGELOG.md in Keep a Changelog format with an Unreleased section #72

Create CHANGELOG.md at the repo root following https://keepachangelog.com/en/1.1.0/. Include the standard header (title Changelog, the note that it follows Keep a Changelog and Semantic Versioning, with links).

Add an ## [Unreleased] section with an ### Added list containing:

• --shout flag that upper-cases the greeting
• --version flag that prints the crate version and exits
• --times N flag that prints the greeting N times (default 1)

The crate version in Cargo.toml is 0.1.0 and nothing has been released, so do not invent a released version section. Do not modify other files.

Definition of done

• grep -q '## \[Unreleased\]' CHANGELOG.md passes.
• grep -q -e '--shout' CHANGELOG.md && grep -q -e '--version' CHANGELOG.md && grep -q -e '--times' CHANGELOG.md passes.