Include new lettuce logo on the README.md

[xpostudio4]

Hi Guys,

This is an strange story for an open source project pull request but here I go... I have been spending the past few weeks studying testing and my wife designed this logo to encourage me and as a gift for the open source project.

I would happily send the original file to the core maintainer if the pull request is accepted. This is a gift of love from her and I hope you like it.