docs: Update svn:ignore documentation to reflect current status.

Hi,

The code that parses svn:ignore properties looks in .svn/dir-prop-base/, which doesn't exist any more (as of 1.7.0, released in 2011).  This patch updates README to reflect this.

All Subversion versions that have a dir-prop-base directory will go out of security support by upstream within a few months, when 1.9.0 is released.  1.7 and later store svn:ignore differently than 1.6 and earlier did; the exact storage format is (and has always been) considered an implementation detail.  I'm happy to provide additional details if interested.