Bug#806697: improvement for README.backtrace

[Harald Dunkel <harald.dunkel@aixigo.de>]

Package: apache2
Version: 2.4.10-10+deb8u3

README.backtrace should mention that the core dump directory
(e.g. /var/cache/apache2) has to be writable for user www-data .
Better use /var/tmp.

Regards
Harri