Update documentation for debugging of Flask apps

[DonJayamanne]

Related to #573

Update launch.json as follows:

		{
			"name": "Python: Flask (0.11.x or later)",
			"type": "python",
			"request": "launch",
			"stopOnEntry": false,
			"module": "flask",
			"pythonPath": "${config:python.pythonPath}",
			"cwd": "${workspaceFolder}",
			"env": {
				"FLASK_APP": "${workspaceFolder}/app.py"
			},
			"args": [
				"run",
				"--no-debugger",
				"--no-reload"
			],
			"envFile": "${workspaceFolder}/.env",
			"debugOptions": [
				"RedirectOutput"
			]
		},

Update with a sample app.py file:

from flask import Flask
app = Flask(__name__)

@app.route('/')
def hello():
    return "Hello World!"

if __name__ == '__main__':
    app.run()

[DonJayamanne]

PR has been submitted microsoft/vscode-docs#1451