[plugins.json] Add cocoapods-dependency-graph

[RY Zheng]

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

{
  "gem": "cocoapods-dependency-graph",
  "name": "CocoaPods Dependency Graph",
  "author": "Rongyan Zheng",
  "url": "https://github.com/EXAMPLE/cocoapods-dependency-graph",
  "description": "A longer description of cocoapods-dependency-graph."
}