Add notes on usage in serverless environments

[dtran320]

• We need to wrap .track calls in a Promise in these environments, which isn't mentioned in this doc.
• Link to the README in the repo for analytics-node for full details

Proposed changes

Added some documentation here in hopes of saving other developers the several hours I unfortunately spent thinking that just calling await closeAndFlush() would suffice for serverless environments—I needed to do the combination of both:

1. Wrapping analytics.track calls in a Promise and awaiting it
2. Instantiate new instances of analytics()— otherwise my big batch of events just never getting sent despite calling closeAndFlush() and having that return. I didn't add this to the documentation since I'm not sure if I'm doing this correctly— any feedback would be appreciated. Happy to update the documentation with this additional information if this is actually necessary. The README seems to suggest it's only necessary if using plugins.

Merge timing

ASAP to prevent any other developers upgrading from the deprecated analytics-node to this library that use it in a serverless environment and previously depended on await analytics.flush().

Related issues (optional)

• flush does not guarantee that all inflight messages are sent analytics-node#309

[forstisabella]

two small changes and this'll be g2g!

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