doc: clarify that Postgres v11 or higher is required

See golang/go#43154, where an older version was missing a function we
use.

Change-Id: I268b5ded629bf10504bb2418843978f21c15f483
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/278292
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
diff --git a/doc/postgres.md b/doc/postgres.md
index 5762608..0aec9cd 100644
--- a/doc/postgres.md
+++ b/doc/postgres.md
@@ -8,8 +8,8 @@
 
 ## Local development database
 
-1. Install PostgreSQL on your machine for local development.
-   It should use the default Postgres port of 5432.
+1. Install PostgreSQL (version 11 or higher) on your machine for local
+   development. It should use the default Postgres port of 5432.
 
    If you use a Mac, the easiest way to do that is through installing
    https://postgresapp.com.