License under AGPL-3.0-only

Ensures anyone running a modified version of C3 as a network service
has to share their changes back, not just users who redistribute the
binary.

- LICENSE: full AGPLv3 text with copyright notice filled in
- Cargo.toml: license = "AGPL-3.0-only"
- README: brief license section

README.md

@@ -27,3 +27,9 @@ docker compose exec ccc /usr/local/bin/ccc add demo /staging/app.js 1.0.0
 (Alternatively `docker cp` a file straight into the container if you'd
 rather not use the staging mount.)
 
+## License
+
+AGPL-3.0-only - see [LICENSE](LICENSE). If you run a modified version
+of this as a network service, you must make the modified source
+available to its users.
+