[R4R] Add rb-jekyll-readme-index

[sodre]

ref: #9315

Checklist

• License file is packaged (see here for an example)
• Source is from official source
• Package does not vend other packages
• Build number is 0
• A tarball (url) rather than a repo (e.g. git_url) is used in your
  recipe (see
  here
  for more details)
• GitHub users listed in the maintainer section have posted a comment confirming they are willing to be listed there

[conda-forge-linter]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipes/rb-jekyll-readme-index) and found it was in an excellent condition.