Add contributer agreement on OpenCMISS website

- Keep it short. 
- Include something along the lines of: You acknowledge that you have the right to contribute your work to the OpenCMISS project and it will be under this license.

Potential options for implementing this:
Plugin to Github - when you do a pull request - it says you haven't signed this, you have to sign it to. 
We thought it might be better to keep a list of 