Add initial contribution guidelines

This PR serves close #1.  The purpose of this PR is to provide the initial contribution guidelines for the [blpapi-http](http://github.com/bloomberg/blpapi-http) project.

There are some items that are left as TODOs to fill-in once more information is available via the wiki and supporting CI infrastructure.  This is also the opportunity to raise any disagreements and to have a constructive conversation on any particular items of interest.