pgsql: Add link to major version release notes at the top of the minor

Log Message:
-----------
Add link to major version release notes at the top of the minor
version ones, to make it clear to users just browsing the notes
that there are a lot more changes available from whatever version
they are at than what's in the minor version release notes.

Tags:
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REL8_2_STABLE

Modified Files:
--------------
    pgsql/doc/src/sgml:
        release.sgml (r1.488.2.17 -> r1.488.2.18)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/release.sgml?r1=1.488.2.17&r2=1.488.2.18)