I know with that an image can be placed in a MD with the MD syntax of either  or . The image from github is delivered as content-type: text/plain, which won't get rendered as an image by your browser.
The correct content-type value for svg is image/svg+xml. So you have to make sure that svg files set the correct mime type, but that's a server issue.
Try it with http://svg.tutorial.aptico.de/grafik_svg/dummy3.svg and don't forget to specify width and height in the tag.