Review and update documentation

[Vitalina-Vakulchyk]

In the main README.md:

• there are no leftovers like deprecated/removed features descriptions;
• add a “Getting Started” section, containing an example with the most basic usage of the gem;
• add information which versions of AR are supported by each major Chewy release;
• explain how Elasticsearch client options is passed to the Elasticseach::Client.new

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the changelog if the new code introduces user-observable changes. See changelog entry format for details.

[dalthon]

Looks good.

I just left a suggestion that you could ignore because I think it might be a matter of taste

[bbatsov]

One goal to have in mind - make sure the documentation features all the information you'd like to see there as a newcomer to the project. That's our bar for success. :-)

[AlexVPopov]

Hey @Vitalina-Vakulchyk, I'll do a couple of additions later today and ping you. :-)

[Vitalina-Vakulchyk]

@toptal-anvil ping reviewers

[bbatsov]

Looks good!