Enrich README with import! doc

[H3nSte1n]

Enriching the section on index manipulation with an example of calling an import bang method.

I have adjusted the section on index manipulations to simplify the search and make the section more coherent and complete.

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• 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.