Add Argo logo to front page

[jlewi]

Lets add the Argo logo to the front page.

Will ask the Argo folks to attach their logo to this issue.

[mukulikak]

Added Argo logo

[abhi-g]

@mukulikak When I click on the image you added to download, it takes me to (https://user-images.githubusercontent.com/25306803/43103633-a5d61dc4-8e83-11e8-9f0e-7ccdbee01eb6.png) which doesn't have "argo" at the bottom, whereas, if I were to download this image as is, it does have argo at bottom. Would you want me to use that linked image, or this one with the "argo" at the bottom.

[abhi-g]

@mukulikak Additionally, I would like to link to Argo website from the logo that I add. I couldn't find any specific website. I am planning to link to "https://blog.argoproj.io/" unless you suggest otherwise.