add images to the Jingo own repository

As you probably already know, you cannot use Jingo to upload, add, edit images to your Markdown documents, but this doesn't mean that you cannot use images from inside your documents altogether. The obvious solution is to provide an URL to an externally hosted image, such as ![A huge bird](https://example.com/a_huge_bird.jpg).

A better approach involves adding images to your local document repository, just using git (and not Jingo). I'd suggest to put your images in a assets directory and not directly together with all the documents – but this is just matter of preferences.

Once your image(s) are into the repository you can refer to them with ![A huge bird](assets/a_huge_bird.jpg).

There is one caveat though: the Preview feature of Jingo's editor won't show you the image. To fix this, just remember to always prefix /wiki to the path of the image. The example before should be written as ![A huge bird](/wiki/assets/a_huge_bird.jpg).

Note: this page has been written in response to issue #153. Thanks to @xu4wang for raising this issue.