chore: updating advice on message processing

[nicholasgriffintn]

Resolves #256

Description:

Updated the advice on message processing to make it clearer how to ensure messages are processed one at a time and that messages can be acknowledged.

Type of change:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Why is this change required?:

It was previously unclear how this functionality works, updating the README makes this clearer.

Code changes:

• Updated the README

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Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codeclimate]

Code Climate has analyzed commit 58d8bf6 and detected 0 issues on this pull request.

The test coverage on the diff in this pull request is 100.0% (80% is the threshold).

This pull request will bring the total coverage in the repository to 94.5% (0.0% change).

View more on Code Climate.