chore(docs): add message hub client example

[joshuachp]

Show how to connect a client to the MessageHub to send and receive data.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.6%. Comparing base (641399f) to head (9334554).
Report is 4 commits behind head on master.

Additional details and impacted files

Files with missing lines	Coverage Δ
src/client.rs	58.3% <ø> (ø)

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[rgallor]

Apart from the comments I've left, I think we should call all the examples using the same convention: up to now we have used examples with non-abbreviated names separated by an underscore, therefore I'd opt to use examples/message_hub_client as the example name. Nevertheless, I would prefer using score-separated names (rather than using the underscore). Maybe we can talk about it separately.

[joshuachp]

Apart from the comments I've left, I think we should call all the examples using the same convention: up to now we have used examples with non-abbreviated names separated by an underscore, therefore I'd opt to use examples/message_hub_client as the example name. Nevertheless, I would prefer using score-separated names (rather than using the underscore). Maybe we can talk about it separately.

I changed to use underscores in the example directory and name.