Merge pull request #30 from willow-ahrens/wma/fill-value

Add language to specify fill value

Author: willow-ahrens

spec/latest/index.bs

@@ -88,6 +88,12 @@ contain the vector's dimension.
 Note: a matrix has shape [`number_of_rows`, `number_of_columns`] regardless of whether
 the format orientation is row-wise or column-wise.
 
+Fill {#key_fill}
+--------------------
+The `fill` key may be present. If the `fill` key is present, it shall have a
+boolean value. If the value is true, it signifies the presence of a `fill_value`
+array, whose single element defines the value at indices not specified
+by the sparse tensor structure.
 
 Format {#key_format}
 --------------------