docs: Update README.md to explicitly state that 'send()' returns a promise

[Ifunanyacollins]

explicitly stating that the send() method returns a promise

Fixes

Checklist

• I acknowledge that all my contributions will be made under the project's license
• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the development branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

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[Ifunanyacollins]

@thinkingserious any clue why build is failing ?

[thinkingserious]

Hello @Ifunanyacollins,

Thanks again for the PR!

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