Add documentation about CSP incompatibility

[tdelmas]

It's important to highlight methods not compatible with a strong Content-Security-Policy (i.e. without unsafe-eval).

In the same way it's documented in https://github.com/d3/d3-dsv#content-security-policy

It should be documented in https://github.com/d3/d3-dsv/blob/master/README.md#csvParse : #65

[mbostock]

I don’t see a need to repeat the CSP documentation that already exists in the README, but feel free to send a PR with suggested edits.