[DOC] Add material-ui dependence

[kpitn]

No description provided.

[fzaninotto]

It's only a problem if you use npm 2 isn't it?

[kpitn]

I don't know if it's related, but I'm usingnpm 2.15.8

[jpetitcolas]

Which version are you using? Because material-ui is already listed as dependencies: https://github.com/marmelab/admin-on-rest/blob/master/package.json#L57

[fzaninotto]

But the tutorial only asks to install admin-on-rest. With npm 2, admin-on-rest's dependencies (including material-ui) are under node_modules/admin-on-rest/node_modules and can't be found by a simple import.

Npm 2 is outdated, and we don't want to spend time supporting it. Instead of making the documentation harder to follow, I suggest that we add node 5 (with npm 3) as required dependency of admin-on-rest.