README With svg demo: TypeError: document.createElement(...).getContext is not a function

I want to use JsBarcode in svg, so i found this in readme file.
i try this step by step, but got some errors
```
const { DOMImplementation, XMLSerializer } = require('xmldom');
const xmlSerializer = new XMLSerializer();
const document = new DOMImplementation().createDocument('http://www.w3.org/1999/xhtml', 'html', null);
const svgNode = document.createElementNS('http://www.w3.org/2000/svg', 'svg');

JsBarcode(svgNode, 'test', {
xmlDocument: document,
});
```
error:
```
Thrown:
TypeError: document.createElement(...).getContext is not a function
at messureText (/Users/longen/temp/node_modules/jsbarcode/bin/renderers/shared.js:77:42)
at calculateEncodingAttributes (/Users/longen/temp/node_modules/jsbarcode/bin/renderers/shared.js:39:16)
at SVGRenderer.prepareSVG (/Users/longen/temp/node_modules/jsbarcode/bin/renderers/svg.js:59:44)
at SVGRenderer.render (/Users/longen/temp/node_modules/jsbarcode/bin/renderers/svg.js:36:9)
at render (/Users/longen/temp/node_modules/jsbarcode/bin/JsBarcode.js:227:11)
at API.render (/Users/longen/temp/node_modules/jsbarcode/bin/JsBarcode.js:206:3)
at JsBarcode (/Users/longen/temp/node_modules/jsbarcode/bin/JsBarcode.js:76:55)
```

env:
node: v10.24.1
jsbarcode@3.11.5

Start with the README SVG example and reproduce the reported Node.js v10 error using the shown xmldom setup. Trace the stack from renderers/shared.js through renderers/svg.js, then verify that the example runs without the getContext TypeError.

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