Update README & CONTRIBUTING files

[cmaumet]

To give a better description of how to contribute to each components and also outline how we comply with INCF requirements for standards.

[cmaumet]

We discussed this point on NIDM call on April 16th, 2018:

• @dbkeator and @khelm liked it!
• Add link to the Tool Smruti is developing in the NIDM-experiment part and the javascript reference API.

[smpadhy]

@cmaumet Link to BrainVerse: https://github.com/ReproNim/brainverse

[cmaumet]

Thanks @smpadhy! The link to Brainverse is now added to the README.