form-field: document new features for 6.0.0

[mmalerba]

• New appearance option feat(form-field, chip): merge new form-field and chip features to master #9762
• Allow type=color fix(input): allow color inputs in mat-form-field #8748
• AutofillMonitor and autofill Sass mixins feat(cdk-input): move input autofill and autosize utils into cdk #9831

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