Exclude test files from the gem package

[amatsuda]

I noticed that the installed redcarpet gem package includes test files that are not usually included in other gems, and misses some documents instead.

So here's a patch that removes the tests and adds three document files.

The reason that I thought why tests are not usually included in other gems is just because the Bundler "new gem" template rejects them. However, this does not necessarily mean that a gem should never include tests. So, feel free to ignore or just close this PR if you're intentionally including them and want to keep shipping gems with tests.

[robin850]

It looks like you are right ; the documentation for #files even states:

Only add files you can require to this list, not directories, etc.

And there is #test_files for that. Thank you very much @amatsuda!