Update readme with addon links

[roborourke]

Don't merge just yet. Just getting ready for the new reduced core plugin and addon library.

[cfoellmann]

When are you completing this transition?
Currently the split codebase is already functioning but getting it all together via composer is a real pain.
Working for now is this composer.json

{
  "name": "private/test-mercator-v2",
  "type": "project",
  "repositories": [
    {
      "type": "git",
      "url": "https://github.com/humanmade/Mercator"
    },
    {
      "type": "git",
      "url": "https://github.com/humanmade/Mercator-Redirect"
    },
    {
      "type": "git",
      "url": "https://github.com/humanmade/Mercator-GUI"
    },
    {
      "type": "package",
      "package": {
        "name": "humanmade/mercator-sso",
        "version": "0.1.0",
        "type": "wordpress-muplugin",
        "source": {
          "url": "https://github.com/humanmade/Mercator-SSO",
          "type": "git",
          "reference": "master"
        }
      }
    }
  ],
  "config": {
    "preferred-install": {
      "*": "dist"
    }
  },
  "require": {
    "humanmade/Mercator": "v2.x-dev as 1.1",
    "humanmade/Mercator-Redirect": "dev-master",
    "humanmade/Mercator-SSO": "0.1.0 as dev-master",
    "humanmade/Mercator-GUI": "dev-master"
  },
  "extra": {
    "installer-paths": {
      "mercator-test/{$name}/": [
        "type:wordpress-muplugin"
      ]
    }
  }
}

I will give the different parts some input but I hope you have got the time to look at them.

[roborourke]

@cfoellmann that's really useful thanks, I kept getting pulled on to other things, been a busy year for the agency. Let me catch up on the db charset issue and then we'll be good to go.