Added actual code example for how to instantiate the Service class

[DMiradakis]

• I've been building on top of the Power BI Javascript SDK for over two years now, and it has been a very painful process for me because this repo is very-poorly documented.
• There was not even a proper code example on how to create a Service class to embed Power BI content.
• PLEASE turn on issues and discussions for this repository. I have asked in the r/PowerBI subreddit before to open issues on this repository, and no one has responded to me.
• I am happy to contribute and help the Power BI community, but my hands are tied when I can't even open a proper issue.
• Please consider adding my code snippet for people who started building apps with the Power BI Javascript SDK before the dedicated JS framework libraries were made (e.g. Power BI Javascript SDK for Vue or React).

[ayeshurun]

Thank you for the enhancement proposed in this pull request. In accordance with best practices, it is advisable to utilize the global variable 'powerbi' rather than initializing a new instance of this class, as proposed in this pull request. A good example can be found in our documentation, which is also referenced in the README file: example.