Add "Getting Started" section to README.md

[gioragutt]

Getting Started

• Show general initialization of Monitored (without specific plugins)
• Basic example of usage: await monitored('my-expensive-method', async () => await myExpensiveMethod())

Getting Started - Prometheus

• Show initialization of the plugin
• Include an example of a curl command of getting the exported metrics
• Example of output AND/OR explanation of metrics sent (with reference link to the plugin code)

Getting Started - StatsD

• Show setup of graphite/w/e with docker/docker-compose
• Show initialization of the plugin
• Give example/link to query that shows the output
• Explanation of metrics sent (with reference link to the plugin code)