Document and use a clearer semver policy

[sourcefrog]

Update https://mutants.rs/stability.html perhaps saying:

We do use semver.

A breaking change (major version) includes:

• Changing the semantics of any config variables
• Adding new mutation patterns, so that existing trees might fail
• Changing heuristics about what is or is not included
• Removing keys from JSON output
• Changing the names of any mutants (so existing exclusions won't work)

Minor changes might include:

• Adding new JSON output
• Adding new config keys or attributes, as long as the default does not change
• UI-only changes

I'm not quite sure if adding new mutations should count as a breaking change or not.

cc @kpreid @sandersaares, feedback welcome!