Added DALL.E 3 to readme.md

[Kshirodram]

Added DALL.E 3 to readme.md as its supported now as per issue #494

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Describe the change
Added DALL.E 3 to readme.md
PR: #494

Provide OpenAI documentation link
Provide a relevant API doc from https://platform.openai.com/docs/api-reference

Describe your solution
It's just a documentation update

Tests
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Additional context
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Issue: #XXXX

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.68%. Comparing base (774fc9d) to head (a66289d).
Report is 12 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #741      +/-   ##
==========================================
+ Coverage   98.46%   98.68%   +0.22%     
==========================================
  Files          24       24              
  Lines        1364     1140     -224     
==========================================
- Hits         1343     1125     -218     
+ Misses         15        9       -6     
  Partials        6        6              

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[sashabaranov]

Thank you!