Add migration and API change doc for 0.x -> 1.0

[Anthropic]

Add documentation for the following issues found during migration processes carried out by users.

• Improve contribution docs #901 contribution docs
• schemaForm.merge regression in 1.0.0-alpha.5 #891 schemaForm.merge no longer requires all parameters
• Add Object.assign polyfill or backwards compatibility library for IE support #869 Add Object.assign polyfill or backwards compatibility library for IE support
• x-schema-form should accept all formats for titleMaps #889 x-schema-form should accept all formats for titleMaps
• sfValidator regression in latest alpha #890 sfValidator API change
• Change to field names in 1-alpha #896 Change to field names

[Anthropic]

All items are covered now, I'll re-open this issue if any more documentation needs pop-up