Add support for building the cli on pull request

[sciabarracom]

Why you need it?

To build the cli when someone contributes a PR and it is accepted.

How it could be?

A "contrib" workflow in github.com/apache/openserverless-cli that

• createa a tag from the version.txt
• trigger a build and a release using the release workflow

Other related information

A partial implementation creating the tag when someone contribute a PR is already available.
However looks like it does not trigger another build for some reasons.
We shoulud either add the trigger or merge the code with the release workflow so it is activated both by pr AND pull requests.

[sciabarracom]

Also add to the README the documentation how to contribute a PR.
Basically all it is needed is to run task tag before contributing will generate a new tag and update the version.txt used by the buid.
Once the version.txt is updated this workflow will propagate the version.txt as a tag and the go releases can build with the right version.