M1/#291: document SMARTMUSIC_* env vars + test DSN

.env.example

@@ -0,0 +1,12 @@
+# Minstrel environment variable overrides.
+# Copy to .env for docker compose, or export before `go run ./cmd/minstrel`.
+# Values shown are sensible defaults for the local docker-compose postgres.
+
+SMARTMUSIC_SERVER_ADDRESS=:4533
+SMARTMUSIC_DATABASE_URL=postgres://minstrel:minstrel@localhost:5432/minstrel?sslmode=disable
+SMARTMUSIC_LOG_LEVEL=info
+SMARTMUSIC_LOG_FORMAT=text
+
+# Set this when running `go test ./...` without -short to exercise the
+# migration test against a running postgres.
+# MINSTREL_TEST_DATABASE_URL=postgres://minstrel:minstrel@localhost:5432/minstrel?sslmode=disable