Processor for extracting README for zip

[zelima]

Originally coming from https://github.com/datahq/datahub-qa/issues/129

Currently, zipped dataset does not contain README (except in the dp.json). We need it in the main directory. To accomplish that we need a simple processor that extracts README from dp.js as a single file. And place this processor before dump.to_zip in planner

Acceptance Criteria

• [ ] zip files have README in it
• [ ] processor is tested

Tasks

• [ ] tests
• [ ] add new processor
• [ ] update planner