Add comment on writable for webdav file source

lib/galaxy/config/sample/file_sources_conf.yml.sample

@@ -15,6 +15,9 @@
   # By default, the plugin will use temp files to avoid loading entire files into memory. 
   # You can change the directory here or omit to use the default temp directory.
   temp_path: /your/temp/path
+  # Set writable to true if you have write access to this source
+  # and want to allow exporting files to it, by default is read only.
+  writable: false
 
 - type: posix
   root: '/data/5/galaxy_import/galaxy_user_data/covid-19/data/sequences/'