docs: Clarify how to use a client with InfluxDB 1.8

[bednar]

Updated README.md with info about InfluxDB 1.8

https://github.com/influxdata/influxdb-client-php/blob/2a776d4f4c159e906c7faaed765f3021cf4408c0/README.md

• CHANGELOG.md updated
• Rebased/mergeable
• Tests pass
• Sign CLA (if not already signed)

[rhajek]

If the Flux query endpoint is disabled by default in 1.8. We should add note how to enable.

[bednar]

If the Flux query endpoint is disabled by default in 1.8. We should add note how to enable.

[codecov-io]

Codecov Report

Merging #22 into master will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##             master      #22   +/-   ##
=========================================
  Coverage     85.31%   85.31%           
  Complexity      271      271           
=========================================
  Files            14       14           
  Lines           613      613           
=========================================
  Hits            523      523           
  Misses           90       90           

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