Document append_positioning flag for NMEA.

diff --git a/xmldoc/formats/options/nmea-append_positioning.xml b/xmldoc/formats/options/nmea-append_positioning.xml
new file mode 100644 (file)
index 0000000..1706e47
--- /dev/null
+++ b/xmldoc/formats/options/nmea-append_positioning.xml
@@ -0,0 +1,5 @@
+<para>
+       When writing NMEA realtime positioning data, append to the 
+       output file instead of truncating it on each successive position
+       fix.
+</para>