document Plot.frame's anchor option (#1243)

Author: Fil

README.md

@@ -2021,6 +2021,8 @@ Decorations are static marks that do not represent data. Currently this includes
 
 The frame mark supports the [standard mark options](#marks), and the **rx** and **ry** options to set the [*x* radius](https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/rx) and [*y* radius](https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/ry) for rounded corners. It does not accept any data or support channels. The default **stroke** is currentColor, and the default **fill** is none.
 
+If the **anchor** option is specified as one of *left*, *right*, *top*, or *bottom*, that side is rendered as a single line (and the *fill*, *rx* and *ry* options are ignored).
+
 #### Plot.frame(*options*)
 
 <!-- jsdoc frame -->