Add feature dependency diagram for the feature workflow wiki page

I would like to add the picture in this PR to the page https://crosswalk-project.org/#wiki/Crosswalk-bug-and-feature-tracking-and-triaging

This PR is to upload the picture to the asset folder, I will edit the wiki separately. It would have been better to do both with the same PR, but I'm not 100% sure of how the picture should be linked and at the moment I have no way to test it locally.