doc: add documentation for schema-type option (#487)

doc/md/schema-fields.md

@@ -51,7 +51,7 @@ The following types are currently supported by the framework:
 - `string`
 - `time.Time`
 - `[]byte` (only supported by SQL dialects).
-- `JSON` (only supported by SQL dialects) - **experimental**.
+- `JSON` (only supported by SQL dialects).
 - `Enum` (only supported by SQL dialects).
 
 <br/>
@@ -139,9 +139,41 @@ func (Pet) Fields() []ent.Field {
 }
 ```
 
+## Database Type
+
+Each database dialect has its own mapping from Go type to database type. For example,
+the MySQL dialect creates `float64` fields as `double` columns in the database. However,
+there is an option to override the default behavior using the `SchemaType` method.
+
+```go
+package schema
+
+import (
+    "github.com/facebookincubator/ent"
+    "github.com/facebookincubator/ent/dialect"
+    "github.com/facebookincubator/ent/schema/field"
+)
+
+// Card schema.
+type Card struct {
+    ent.Schema
+}
+
+// Fields of the Card.
+func (Card) Fields() []ent.Field {
+	return []ent.Field{
+		field.Float64("amount").
+			SchemaType(map[string]string{
+				dialect.MySQL:    "decimal(6,2)",   // Override MySQL. 
+				dialect.Postgres: "numeric",        // Override Postgres.
+			}),
+	}
+}
+```
+
 ## Default Values
 
-**Non-unique** fields support default values using the `.Default` and `.UpdateDefault` methods.
+**Non-unique** fields support default values using the `Default` and `UpdateDefault` methods.
 
 ```go
 // Fields of the User.