Update README.md

[cymulatereouven]

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[cymulatereouven]

Pull Request Description
Summary
This PR updates the documentation to include specific instructions for building node-rdkafka on Mac M1 systems with OpenSSL. Previously, the documentation only provided instructions for Mac OS High Sierra and Mojave users. The new section ensures compatibility with M1 chip architecture by adjusting the environment variables to correctly link OpenSSL.

Changes:
Added OpenSSL linker flags for Mac M1 in the documentation.
The following environment variables should be used on Mac M1 systems:
sh
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export CPPFLAGS=-I/opt/homebrew/opt/openssl@3/include
export LDFLAGS=-L/opt/homebrew/opt/openssl@3/lib
Context
Mac M1 users encountered issues when building node-rdkafka due to differences in OpenSSL library locations. This update ensures proper linking for users working on M1 systems with Homebrew-installed OpenSSL.