Signet version 0.9.18.1 release notes

### New features

### Enhancements

- enable automated CI builds of .deb packages
- added PGP key to repo and fingerprint to README
- clean up generated signet.1.gz file on make clean
- added keywords to launcher shortcut to be standards compliant
- added code of conduct and guide to contributing

### Bug fixes

- corrected typo in widget string
- corrected typo in warning message string
