Explain how changelog is generated on Readme

[danielo515]

Hello,

Understanding how the changelog is generated would help users understand why they are getting the results they saw and how they can improve their workflow to take maximum advantage of lerna-changelog.

For example, I'm getting the following output:

## Unreleased (2017-01-04)

#### Committers: 0

And I can't understand why. Having at least a friendly explanation of how changes are fetched would help a lot.

Regards