Fix for #128: Documentation enhancement for Github Actions

[jofas]

Hi,

this PR enhances the example for use of this project with Github Actions. The problems I faced when I copy and pasted the example into my Github Action are described in issue #128.

Adding the --hide-footer flag makes sure that no unnecessary auto-commit is generated by the Gitub Action. For example, if you were to change the content of foo.md, but would leave the structure of its headlines intact, there is no need to generate a new TOC. gh-md-toc adds a footer comment to your TOC that reads something like <!-- Created by <user> at <datetime> -->. This changes every time gh-md-toc is executed at a different datetime. Hiding the footer makes running gh-md-toc idempotent, if the headers in foo.md do not change. This removes auto-commits by the Github Action even when the TOC does not change.

Adding the rm gh-md-toc line to the action removes the gh-md-toc script from the repository again, so it won't be added to the repo by the auto-commit of the Github Action.

[ekalinin]

Thanks!