Require Joi module in the example of README doc

[abbassiddiqi]

Joi is being used in the example, but it is not imported/required

id: Joi.string().min(5).required(),

I checked the package code to see if you are exporting the Joi module, but there only a single function middleware was exported. So I added the require statement for Joi.

[coveralls]

Coverage remained the same at 100.0% when pulling a3b5d6a on abbassiddiqi:require-joi into f044ddf on oxyno-zeta:master.