[plugins.json] Add cocoapods-monorepo

[menttofly]

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

{
  "gem": "cocoapods-monorepo",
  "name": "CocoaPods Monorepo",
  "author": "menttofly",
  "url": "https://github.com/menttofly/cocoapods-monorepo.git",
  "description": "`cocoapods monorepo` will make the dependencies that under specified directory into development pods."
}