Added description for pre-stable public API versioning

When reading the description for major version zero I came across the line "Anything may change 
at any time." I would like to create a more clear explanation of how versioning should take place before the first stable version of the public API so that developers can have meaning behind their pre-stable versioning.

This is especially an issue in node.js development as by default saved modules will pick up new patch versions but not new minor versions. If developers don't properly version their modules in a pre-stable state they can potentially break when users pick up new patch versions.

To define this pre-stable state I have proposed moving the definition for major and minor changes to the right, meaning that API-breaking changes result in a minor version being incremented and both backwards-compatible functional additions as well as bug fixes result in a patch version being incremented.