Document workaround for guzzle6-adapter 1.x

Problem/Motivation

We depend on php-http/guzzle6-adapter ^2.0. There could be some modules that depend on an older version which would cause a conflict when installing. We found this first with geocoder module which has been since fixed in #3160832: Support for v2.0 of php-http/guzzle6-adapter . There could be other similar modules.

Proposed resolution

Add documentation to resolve this conflict using composer aliasing as there is very littler breaking change in guzzle6-adapter between 1.0 and 2.0.