Add an erratum for svn_fs_props_changed and svn_fs_contents_changed.

* notes/api-errata/1.9
  (): New folder.  It's the first erratum for 1.9.

* notes/api-errata/1.9/fs001.txt
  (): New file.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1618880 13f79535-47bb-0310-9956-ffa450edef68