Improve Documentation on adding geocoder-php Providers

Well no, which one would you provide? You cannot know which provider the user will need. There is no default provider in the geocoder-php/geocoder library.

By doing this it would make things needlessly complex for the end user. We would need a way to add this to the composer dependencies, but we cannot add it to the "require" dependencies since then the end user would not be able to remove it any more from their project.

Even if we find a way to do this then the end user now will have to do two tasks instead of one: they will have to add the provider of their choice AND remove the default one that we provide.

Thanks (probably) to additional documentation (still to be produced) he should realize the need to add the needed Geocoder provider

It's already documented in the Readme: See https://cgit.drupalcode.org/geocoder/tree/README.md?h=8.x-3.x#n39

Yes we could improve the documentation a lot, it is just some very brief instruction in the install steps now, it's easy to miss.

Currently it says this:

* Choose the [Geocoder Provider](https://packagist.org/providers/geocoder-php/provider-implementation)
  you want to use and also add it as a required dependency to your project. For
  example if you want to use Google Maps as your provider:
  ```$ composer require geocoder-php/google-maps-provider```

We could provide a section in the project page, and in the online documentation as well outlining some of the popular providers.

I reviewed and updated(Improved) the coding standards within the documentation.

I also tried reviewing the step-by-step approach as requested for this issue but this module has got lots of requirements(dependencies) which I suggest still needs to be worked on.