Remove “reference” from README #10675

As @richvdh put it in this matrix.org FAQ issue, it is unfair to call synapse a reference implementation. Yet that’s what the README states.

Synapse is a reference "homeserver" implementation of Matrix from the core
development team at matrix.org, written in Python/Twisted.

In the FAQ, we went with

Synapse is a Matrix "homeserver" implementation developed by the matrix.org core team, written in Python 3/Twisted. It is intended to showcase the concept of Matrix and let folks see the spec in the context of a codebase and let you run your own homeserver and generally help bootstrap the ecosystem.

Given how similar it is to the current paragraph in the README, I believe this paragraph was at first copy-pasted from one place to the other. I suggest we copy-paste back from the FAQ to the README and can even provide a PR if that solution is approved.

FWIW in another place in the FAQ we also have:

Synapse is the most well-distributed homeserver