Add a note re necessary software

README.md

@@ -49,6 +49,16 @@ enough libc to run golang binaries.
 
 To run a more complete system, you also need to have docker compose installed.
 
+### Database
+
+To connect to the database, you need psql (PostgreSQL client, for
+`./diachron/common.d/db`)
+
+- macOS: `brew install libpq` (and follow the caveat to add it to your PATH),
+  or `brew install postgresql`
+- Debian/Ubuntu: `apt install postgresql-client`
+- Fedora/RHEL: `dnf install postgresql`
+
 ### Development requirements
 
 To hack on diachron itself, you need the following: