Document transitioning from 1.x to 2.0 client libraries

[sanderson]

InfluxDB 1.8 ships with a 2.0 compatible /write endpoint that lets users use the new 2.0 client libraries to write to InfluxDB 1.8 and Cloud 1. We need to write a guide or guides for transitioning from the 1.x to the 2.0 client libraries.

Notes & questions

• These may need to be language/client specific
• What are the gotchas of the new clients?
• Are there new concepts that 1.x users need to understand to successfully use the new clients.

[kelseiv]

close per #2757