Document upgrading the Postgres database files.

I ran into some trouble trying to upgrade my Dockerized setup to 0.18:

fw-postgres_1      | 2019-01-27 18:09:38.287 UTC [1] FATAL:  database files are incompatible with server
fw-postgres_1      | 2019-01-27 18:09:38.287 UTC [1] DETAIL:  The data directory was initialized by PostgreSQL version 9.4, which is not compatible with this version 11.1 (Debian 11.1-1.pgdg90+1).

It looks like the changes in !529 (merged) didn't come with any upgrade path from the old Postgres for people like me who decided to take all the changes in 0.18's docker-compose.yml on top of an 0.17 installation.

I've documented what I did to upgrade the containerized Postgres's database, since upgrading Postgres is somehow not natively supported by the official Postgres docker images, because you need both the old and new Postgres versions installed side by side.

I'm pointing people at a third party set of containers designed to do the database upgrade for you between any two Postgres versions. They worked for me.