Added logo files and to readme - #71

I think there is a problem with the "Open in Matlab Online" include code in README.md
Matlab File Exchange renders the Readme very poorly:
https://de.mathworks.com/matlabcentral/fileexchange/?term=ifdiff&s_tid=ta_fx_prevsearch

I think there is a problem with the "Open in Matlab Online" include code in README.md Matlab File Exchange renders the Readme very poorly: de.mathworks.com/matlabcentral/fileexchange?term=ifdiff&s_tid=ta_fx_prevsearch

Seems to only be an issue in the search preview. It looks fine on the main file exchange page (https://www.mathworks.com/matlabcentral/fileexchange/184589-ifdiff-the-toolkit-for-odes-with-state-dependent-switches?s_tid=srchtitle).
I don't think there's a way for the button to render correctly in the text preview since it's essentially an image. Although it might be better to just remove the button from the top of the README, since we already have it at the beginning of the installation section anyways.