I know with that an image can be placed in a MD with the MD syntax of either ![Alt text](/path/to/img.jpg)
or ![Alt text](/path/to/img.jpg "Optional
I contacted GitHub to say that github.io-hosted SVGs are no longer displayed in GitHub READMEs. I received this reply:
We have had to disable svg image rendering on GitHub.com due to potential cross site scripting vulnerabilities.
This will work. Link to your SVG using the following pattern:
https://cdn.rawgit.com/<repo-owner>/<repo>/<branch>/path/to.svg
The downside is hardcoding the owner and repo in the path, meaning the svg will break if either of those are renamed.