add docstring for grdvolume

Author: willschlitzer

pygmt/src/grdvolume.py

@@ -26,6 +26,14 @@
 @kwargs_to_strings(C="sequence", R="sequence")
 def grdvolume(grid, data_format="a", **kwargs):
     r"""
+    Read a 2-D grid file and calculate the volume contained below the surface
+    and above the plane specified by the given contour (or zero if not given)
+    and return the contour, area, volume, and maximum mean height
+    (volume/area). Alternatively, a range of contours can be specified to
+    return the volume and area inside the contour for all contour values.
+    Using **-T**, the contour that produced the maximum mean height
+    (or maximum curvature of heights vs contour value) is returned as well.
+
     {aliases}
 
     Parameters