Publish crate from CI and create tags

[mondeja]

• The crate is currently manually released. This is tedious and error prone.
• No GitHub releases nor tags are created. This is very bad for operative system packages maintainers. They expect to package libraries using git archive from tags.

Solution:

• Release from CI when pushing tags using a crates token. Add instructions to CONTRIBUTING.md file describing how to release a new version.
• Generate releases on CI from tags. Add .gitattributes file to skip some files from being archived.