add CONTRIBUTING.md

* define the Copyright Statement signing process
* describe the acceptance process
* clarify how pull requests should look like

fixes #28
Contract Definition

Results of discussions in #40, #19, #37, #41 and #46
register git commit email in CopyrightWaivers
Merge pull request #50 from rkuhn/wip-CONTRIBUTING-rk

add CONTRIBUTING.md
Merge pull request #55 from benjchristensen/contract-readme

Contract Definition
Adding benjchristensen
document rationale for void return of subscribe()

fixes #43