pgsql: Add new FAQ item: <H3 id="item1.15">1.15) How do I

Log Message:
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Add new FAQ item:

<H3 id="item1.15">1.15) How do I unsubscribe from the
PostgreSQL email lists? How do I avoid receiving duplicate
emails?</H3>

Modified Files:
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pgsql/doc:
FAQ (r1.450 -> r1.451)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/FAQ?r1=1.450&amp;r2=1.451)
pgsql/doc/src/FAQ:
FAQ.html (r1.406 -> r1.407)
(http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/doc/src/FAQ/FAQ.html?r1=1.406&amp;r2=1.407)