Document mediapipe

[raffaelemancuso]

I think that README.md lacks the fact that the software supports mediapipe.

It's the best filter for me.

[allo-]

That's because I would like better integration than the current one, but didn't had the time to implement it, yet. Now it's more like something you can use at your own risk.
Personally I get blocky artifacts (Image visible but one can see square contours with different color intensity) but others seem not to have the problem. What's missing are checks which filters are working as some will fail when only segmentation is available.
Related issue: #75