Update gcp.rst

Suggestions are in response to errors discussed in this thread, https://community.opendronemap.org/t/error-processing-stopped-because-of-strange-values-in-the-reconstruction/21309/9

source/gcp.rst

@@ -28,7 +28,8 @@ The format of the GCP file is simple.
 
 	 * The first line should contain the name of the projection used for the geo coordinates. This can be specified either as a PROJ string (e.g. ``+proj=utm +zone=10 +ellps=WGS84 +datum=WGS84 +units=m +no_defs``), EPSG code (e.g. ``EPSG:4326``) or as a ``WGS84 UTM <zone>[N|S]`` value (eg. ``WGS84 UTM 16N``)
  	 * Subsequent lines are the X, Y & Z coordinates, your associated pixels, the image filename and optional extra fields, separated by tabs or spaces:
- 	 * Elevation values can be set to "NaN" to indicate no value
+ 	 * Avoid setting elevation values to "NaN" to indicate no value. This can cause processing failures. Instead use 0.0
+         * Similarly decreasing the no. of digits after the decimal place for `geo_x` and `geo_y` can also reduce processing failures. 
  	 * The 7th column (optional) typically contains the label of the GCP. 
 
 GCP file format::