Create an InVEST Contributor License Agreement

[phargogh]

After some discussions with Stanford's OTL and with the NatCap software team, we have settled on a contributor license agreement that:

• clarifies that a person has the right to contribute changes to the project
• grants NatCap the ability to release derivative and collective works under a different license in the future, provided approval of NatCap leadership.

There is also the nifty open-source project https://github.com/apps/cla-bot that can help automate the verification of CLAs during pull requests.

Here's the overall sequence I think we should implement:

• make public the CLA, having interested folks email our new legal address with their assent
• Improve the form so that verification is stored in a known place
• Integrate the CLA-bot in the InVEST and InvEST UG repos to automate checking of CLA signatures

[phargogh]

We now have a (private) repository at https://github.com/natcap/invest-cla, where a github pages deployment redirects to the AWS-hosted signing application. We have not yet integrated the CLA-bot into appropriate repositories.