Add a license file

[keithing]

Hi! I see in your readme and here that his project has an MIT license. Would you mind adding a license file just to make that crystal clear? I'm basing this off the recommendation here:

As a best practice, we encourage you to include the license file with your project.

[chriddyp]

Just added a license. Thank you @keithing !

[keithing]

Quick turnaround! Thanks!