Submitting.md: add a TiKZ example

[pedramardakani]

Until now, readers were not informed about their access to the powerful TiKZ library inside their own paper. This might lead them to using rasterized graphics inside their publication which cannot compete with the infinite resolution of vector graphics.

Also, some might try hacking different methods to produce the illustrations and inject them inside the 'paper.md' file with the markdown syntax. This leads to inconsistent typesetting inside the final paper.

With this commit, I've put a TiKZ example taken from the pgfmanual documentation for a demonstration. This example enjoys the same typesetting as the rest of the paper. For instance, the figure caption has the same font as the body.

[pedramardakani]

Hi all 👋

The openjournals/inara docker image compiled the paper successfully. Hopefully this will encourage using the TiKZ library inside the paper easily without the need to find a hack.

Best, Pedram