Add doc on how to start multiple debuggers in same vscode window

[alexkli]

Originally reported and contributed by @moritzraho

The doc states that only one debugger can be started per vscode window, which is not true anymore.
In fact, vscode debug config has the concept of compounds which allows to aggregate multiple configurations.
This also enables debugging for sequences OOTB (i.e. by starting a wskdebug instance for all the actions of a sequence). Which makes wskdebug even more useful :)
Here is a launch.json example that exposes a config starting 2 wskdebug instances:

{
  "configurations": [
    {
      "type": "node",
      "request": "launch",
      "name": "mypackage/action1",
      "runtimeExecutable": "wskdebug",
      "args": [
        "tmypackage/action1",
        "${workspaceFolder}/action1.js"
      ],
      "localRoot": "${workspaceFolder}/actions/hello",
      "remoteRoot": "/code",
      "outputCapture": "std"
    },
    {
      "type": "node",
      "request": "launch",
      "name": "mypackage/action2",
      "runtimeExecutable": "wskdebug",
      "args": [
        "mypackage/action2",
        "${workspaceFolder}/action2.js"
      ],
      "localRoot": "${workspaceFolder}/actions/hello2",
      "remoteRoot": "/code",
      "outputCapture": "std"
    }
  ],
  "compounds": [
    {
      "name": "All actions",
      "configurations": [
        "mypackage/action1",
        "mypackage/action2"
      ]
    }
  ]
}

[alexkli]

Fixed by #7.