Add user logo guidelines

[el1f]

We should have some guidelines in place to allow for an easier time when adding logos to the home page. Nothing spectacular, just a paragraph on the Readme or somewhere else giving details on how to structure the SVG and the sizes to use in order to keep consistency in the carousel without resorting to hacks involving margins, paddings, transforms and so on. We may also want to use SVGO to optimize the SVGs.

[kwaimind]

I can look into this if you want to assign me. Will add my process from my most recent PR and get some input from other collaborators too.

[el1f]

Go for it @kwaimind! Looking forward to seeing what you come up with :D