Update section 'Create a new release branch' of the README

[eine]

Since a bundler (webpack) is used now, creation of new release branches is easier. Only four files need to be published (README.md, action.yml, main.js, and main.js.map). Instructions should be updated accordingly. I.e., there is no need to remove items explicitly (git rm -rf *.json *config.js *.lock .github .gitignore .v0 __tests__ src examples cli.sh); it should be remove everything except 'dist'.