added 2.6 changelog file and listed 2.6.1 changes. See DOCS-3251.

Signed-off-by: Sam Kleinman <samk@10gen.com>
removed a ticket that does not need to be in the changelog (internal only).

Signed-off-by: Sam Kleinman <samk@10gen.com>
added SERVER-13691 to issue list.

Signed-off-by: Sam Kleinman <samk@10gen.com>
DOCS-3285: changelog edits