docs: added instructions for obtaining OpenAI API key to README

[vvatanabe]

@sashabaranov

Describe the change
Adding instructions on how to obtain the OpenAI API key in README solves the following problems:

1. Ease and Convenience: New developers can quickly understand how to obtain the API key, enabling them to start using the library faster.
2. Preventing Misunderstandings: Without clear instructions, developers could potentially follow incorrect steps, leading to wasted time or potential security risks.
3. Reducing Queries: By providing clear instructions, there's less need for developers to ask questions or make inquiries about obtaining the API key, freeing up time for the maintainers of the library.

Tests
None.

Additional context
Issue: #73

[codecov]

Codecov Report

Merging #421 (e9eecae) into master (177c143) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #421   +/-   ##
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  Coverage   96.95%   96.95%           
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  Files          17       17           
  Lines         690      690           
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  Hits          669      669           
  Misses         15       15           
  Partials        6        6           

[sashabaranov]

Great idea to put this into the README, thank you! I suggest we put it just before Other examples: — installation is basic usage is probably more important to keep on the top of the README

[vvatanabe]

@sashabaranov Thanks for review! I fixed it 👍