[HACKERS] Release Note changes

Migration to Version 10

"A dump/restore using pg_dumpall, or use of pg_upgrade, is required
for those wishing to migrate data from any previous release."

This isn't true and is seriously misleading since pglogical and other
proprietary tools exist that were designed specifically to allow this.
Suggested additional wording would be...

"If upgrading from a 9.4 server or later, external utilities using
logical decoding, such as pglogical or proprietary alternatives, can
also provide an alternate route, often with lower downtime."

Our docs mention pglogical already, so don't see an issue with
mentioning it here.

-- 
Simon Riggs                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services