Improve README.md for how to enable search

[stephenjayakar]

I initially was going to make a post about how search doesn't work out of the box, and I couldn't figure out how to fix it. However, after looking at your actual website, I found this block that ended up fixing search.

[outputs]
  home = ["html", "searchIndex"]

[outputFormats]
  [outputFormats.searchIndex]
    mediaType = "application/json"
    baseName = "search-index"
    isPlainText = true
    notAlternative = true

I'm new to hugo so it's possible this was obvious to you, but it would be really helpful to add this to your README