Write a document on how datadog-service works under the hood

[vadasambar]

Problem statement.
Right now we don't have any document which explains how datadog-service works. This means to understand the workings of the service you need to read the code. It'd be good if we had a document which explains how the datadog-service works. This'd help users in understanding the service, setting expectations (which in turn might create new feature/enhancement requests)

Describe the solution you'd like
I'd like a markdown document with a link to this document in the README explaining:

1. What happens when datadog-service receives an event from testing-service (when the evaluation period is not static)
2. What happens when datadog-service receives an event from the shipyard (when we specify a static evaluation period)
3. How does datadog-service fetch the data from datadog
   ^ we can expand on it as we go.

Describe alternatives you've considered
Another option could be making a video explaining how the service works but that could be one of the things to do for future.

TODO

• Create a document in the root of the repo called how-does-it-work.md (we can change the name later if needed)
• Create an Index in the README below the main heading and add an item called How does it work with link to our markdown (you will have to create the index with other existing headings while doing this)