Defining a field as unique in my entity (but not primary key)

Question

I use Entity Code first in my solution. Lets say I already have a primary key in my table (projectID) but I would like another field to be unique (SlugName). How can I achieve this?

Here is my entity:

[Key]
public int ProjectID   { get; set; }
public string Name     { get; set; }
public string SlugName { get; set; }

Thanks.