Document that the vendor directory must be excluded

[AnyOldName3]

This line https://github.com/jeffreytse/jekyll-deploy-action/blob/master/entrypoint.sh#L22 causes Bundler to install gems to the vendor subdirectory of the working directory so gems can be cached. However, it also means Jekyll will attempt to include files from gems in the site, potentially leading to errors like https://hjerpbakk.com/blog/2020/05/25/your-site-could-not-be-built for a site that built just fine outside of this action.

Documenting that the vendor directory needs adding to the site's exclude list would be helpful for avoiding and diagnosing this kind of problem.

[jeffreytse]

Hi @AnyOldName3

I'm so sorry for the late reply as my busyness. Thanks for your kind advice, I will try to work on it ASAP.

Thanks & Regards