Add tutorial for exporting data from InfluxDB and importing into CrateDB

[amotl]

Hi there,

I conducted some experiments the other day about how to export data from InfluxDB and import into CrateDB. This README is the result from those experiments.

The outcome was that the influxdb-write-to-postgresql program croaked with ERROR: Relation 'pg_indexes' unknown, when trying to import data into CrateDB. Therefore, I created crate/crate#11697.

With kind regards,
Andreas.

/cc @proddata, @hammerhead, @seut

[amotl]

The problem outlined above was resolved on 8 Sep 2021 already, see crate/crate#11697 and crate/crate#11706. So, we need to get back to this in order to check if the example can make some progress.