Add contributor list

As discussed at incf-nidash/nidm#111, this pull request add a contributor list to the specification documents.

Are listed as contributors in `NIDM-Results`,  `NIDM-Overview` and  `NIDM-Primer`  all the authors of our [Neuroinformatics 2014 NIDM abstract](https://docs.google.com/document/d/1Rwv_fiC0Re7eXExSX6sjJsNqX1WlSMQub_zpNyI1srQ/edit?usp=sharing).

@nicholsn, do you think you could similarly include a contributor list for `nidm-experiment` and `nidm-datasetdescriptors`?