Problem: patch requirements are not clearly exposed
Problem: patch requirements are not clearly exposed

When one sends a pull request to es4j, the contributing
documentation linked does not cover main requirements to
a patch.

Solution: update contributing guidelines to include such
requirements.

yrashk/es4j,eventsourcing/es4j,eventsourcing/es4j,eventsourcing/es4j,yrashk/es4j