Update README.md

Author: laurentkempe

README.md

@@ -3,7 +3,7 @@
 
 Git Diff Margin displays live Git changes of the currently edited file on Visual Studio margin and scroll bar.
 
-* Supports Visual Studio 2012 through Visual Studio 2017
+* Supports Visual Studio 2012 through Visual Studio 2019 Preview 1
 * Quickly view all current file changes on
     * Left margin
     * Scroll Bars in map and bar mode with and without source overview