pgsql-server: Update the in-code documentation about the transaction

Log Message:
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Update the in-code documentation about the transaction system. Move it
into a README file instead of being in xact.c's header comment.
Alvaro Herrera.

Modified Files:
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pgsql-server/src/backend/access/transam:
xact.c (r1.175 -> r1.176)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/xact.c.diff?r1=1.175&r2=1.176)

Added Files:
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pgsql-server/src/backend/access/transam:
README (r1.1)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/transam/README?rev=1.1&content-type=text/x-cvsweb-markup)