Need a doc on Type-to-DataView schema mapping

[Zruty0]

We should have a doc that describes exactly how we go from

        public class IrisData
        {
            [Column("0")]
            public float Label;

            [Column("1")]
            public float SepalLength;

            [Column("2")]
            public float SepalWidth;

            [Column("3")]
            public float PetalLength;

            [Column("4")]
            public float PetalWidth;
        }

to the schema of the data view. It should cover:

• Why field types are important, and how they are used
• What exactly is ColumnAttribute, ColumnNameAttribute
• Handling of vectors and VectorTypeAttribute
• Handling of key types and KeyTypeAttribute
• SchemaDefinition as a means of runtime schema hints.
• Limitations / what can not be done.