vg/draw: add package level documentation

Fixes #678.

Signed-off-by: Sebastien Binet <[email protected]>

Author: sbinet

Diff for: vg/draw/doc.go

@@ -0,0 +1,6 @@
+// Copyright ©2021 The Gonum Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package draw provides types and functions to draw shapes on a vg.Canvas.
+package draw