refs #13: Added GitHub action to automate release creations based on git tags.

[stickgrinder]

This contribution should solve issue #13 (carenalgas/popochiu#13) entirely.

This introduces an empty build folder necessary to the automation (just to keep things tidy, can be removed if it seems ugly), plus a release-notes folder that will hold a history of release notes in markdown format.
The core is a short github action workflow which does the trick.

Here is the new release workflow explained:

When we are ready for a release:

• We decide for a release number in the format vX.Y.Z (as an example say we want to release v1.9.0). Note: alpha, beta and rc suffixes are all supported, so f.e. v1.9.0-beta12 is a valid release tag.
• A new release notes markdown file should be made available in the release-notes directory, named after the release number, ex: v1.9.0.md.
• The file must be committed to the main branch.
• A new tag is created, in the format vX.Y.Z, so in our example v1.9.0
• To trigger the release, just issue git push && git push --tags, always on the main branch

The system will create a new release, with release notes read from the provided file (empty in case no file is provided, may be edited afterwards in case we left it our for some reason).

The new issue will be published and marked as latest, so it's always reachable at releases/latest URI.
Making Godot AssetsLib point to the latest one, should solve the update part of the issue without messing with AssetsLib API.

[stickgrinder]

Added support for alpha, beta, rc tags (see PR description) + Added LICENSE file in the final distribution package; also removed LEEME.md which doesn't seem to be in the dev repo.

Adding it back is pretty simple, can be done when we merge repos and close the dev one.

[mapedorr]

I think we can test this precisely with v1.9.0, which according to our last meeting, could have the update to the navigation system that solves the deprecation warning that appeared in Godot 3.5.