Add a file that explains default gems

[janlelis]

Based on the current README and the discussion on the ruby bug tracker I have created this documentation file, which could serve as a template for other default gems.

What do you think about this approauch? What else could be in there?

[stomar]

IMO the essential information should be directly included in a subsection of the README, not a separate file.

[yuki24]

I second @stomar. When it's in a separate file it's a bit harder to notice.

[marcandre]

Thanks for the PR. I couldn't find much differences with the existing README.

The stdgems.org project looks cool but feels like too much information for most people imo. I'd rather keep it simple in the Changelog, at least for now.

I'm closing this, feel free to create a PR with direct changes to the README for improvements.

[stomar]

Afterthought regarding finding out the used version: if this would be included in the README for this or all default gems, I personally would favor an „offline“ way to do it, like gem list, or a VERSION constant, or similar.