Update README

Author: mbostock

README.md

@@ -720,6 +720,8 @@ The following channels are required:
 * **x2** - the ending horizontal position; bound to the *x* scale
 * **y2** - the ending vertical position; bound to the *y* scale
 
+For vertical or horizontal arrows, the **x** option can be specified as shorthand for **x1** and **x2**, and the **y** option can be specified as shorthand for **y1** and **y2**, respectively.
+
 The arrow mark supports the [standard mark options](#marks). The **stroke** defaults to currentColor. The **fill** defaults to none. The **strokeWidth** and **strokeMiterlimit** default to one. The following additional options are supported:
 
 * **bend** - the bend angle, in degrees; defaults to zero
@@ -978,6 +980,8 @@ The following channels are required:
 * **x2** - the ending horizontal position; bound to the *x* scale
 * **y2** - the ending vertical position; bound to the *y* scale
 
+For vertical or horizontal links, the **x** option can be specified as shorthand for **x1** and **x2**, and the **y** option can be specified as shorthand for **y1** and **y2**, respectively.
+
 The link mark supports the [standard mark options](#marks). The **stroke** defaults to currentColor. The **fill** defaults to none. The **strokeWidth** and **strokeMiterlimit** default to one.
 
 The link mark supports [curve options](#curves) to control interpolation between points. Since a link always has two points by definition, only the following curves (or a custom curve) are recommended: *linear*, *step*, *step-after*, *step-before*, *bump-x*, or *bump-y*. Note that the *linear* curve is incapable of showing a fill since a straight line has zero area. For a curved link, you can use a bent [arrow](#arrow) (with no arrowhead, if desired).