Create CONTRIBUTING guide and contracts

[chendaniely]

• Need to create files for contributing
• Need to also create a contract for people who want to be maintainers (Update + status on repository #33)
• Contract for people who want to work on the pyopensci submission Document package and submit to PyOpenSci #14

[achimgaedke]

@chendaniely - how can we help with this? Are there similar contracts we could use as template?