[plugins.json] Add cocoapods-foundation-headers

[Keith Smiley]

Please add the following entry to the plugins.json file:

{
  "gem": "cocoapods-foundation-headers",
  "name": "CocoaPods Foundation Headers",
  "author": "Keith Smiley",
  "url": "https://github.com/keith/cocoapods-foundation-headers",
  "description": "A CocoaPods plugin for using Foundation instead of UIKit/Cocoa in generated headers"
}

[Keith Smiley]

Submitted #83 as recommendation from #79 (comment)