Document Client Key as Required Field for UploadAPI

[jpalmour]

In https://github.com/careerbuilder/CandidateDataServicesDocumentation/blob/master/UploadAPI.md, client_key is not listed as a field, despite it being a required field.

[jpalmour]

Also, in

HTTP POST
Accept: application/json;version=2.0 
https://api.careerbuilder.com/corporate/mysupply/upload

Documentation should addContent-Type: application/json. Both headers appear to be necessary in order to get a 202 response.

[thomasmahoney]

thanks @jpalmour - #11 was just merged to document the required client_key field. we'll look into the Content-Type issue soon.