Add documentation for audioContext option

[Elias Meire]

Mp3MediaRecorderconstructor has following signature:

constructor(stream: MediaStream, options?: { audioContext?: AudioContext });

where the user of the library has the possibility to pass in their own audioContext.

This should be documented in the README.

[Elias Meire]

🎉 This issue has been resolved in version 4.0.0 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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