shell: add a package godoc note for Windows paths

As suggested in mvdan#356, since this can be a common misconception.

shell/doc.go

@@ -3,4 +3,12 @@
 
 // Package shell contains high-level features that use the syntax, expand, and
 // interp packages under the hood.
+//
+// Please note that this package uses POSIX Shell syntax. As such, path names on
+// Windows need to use double backslashes or be within single quotes when given
+// to functions like Fields. For example:
+//
+//     shell.Fields("echo /foo/bar")     // on Unix-like
+//     shell.Fields("echo C:\\foo\\bar") // on Windows
+//     shell.Fields("echo 'C:\foo\bar'") // on Windows, with quotes
 package shell