remove outdated documentation in README

README.md

@@ -1,44 +1,7 @@
-Command gencodec generates marshaling methods for struct types.
+Command gencodec generates marshaling methods for Go struct types.
 
-When gencodec is invoked on a directory and type name, it creates a Go source file
-containing JSON and YAML marshaling methods for the type. The generated methods add
-features which the standard json package cannot offer.
+The generated methods add features which json and other marshaling packages cannot offer.
 
-	gencodec -dir . -type MyType -out mytype_json.go
+	gencodec -dir . -type MyType -formats json,yaml,toml -out mytype_json.go
 
-### Struct Tags
-
-All fields are required unless the "optional" struct tag is present. The generated
-unmarshaling method returns an error if a required field is missing. Other struct tags are
-carried over as is. The standard "json" and "yaml" tags can be used to rename a field when
-marshaling.
-
-Example:
-
-	type foo struct {
-		Required string
-		Optional string `optional:""`
-		Renamed  string `json:"otherName"`
-	}
-
-### Field Type Overrides
-
-An invocation of gencodec can specify an additional 'field override' struct from which
-marshaling type replacements are taken. If the override struct contains a field whose name
-matches the original type, the generated marshaling methods will use the overridden type
-and convert to and from the original field type.
-
-In this example, the specialString type implements json.Unmarshaler to enforce additional
-parsing rules. When json.Unmarshal is used with type foo, the specialString unmarshaler
-will be used to parse the value of SpecialField.
-
-	//go:generate gencodec -dir . -type foo -field-override fooMarshaling -out foo_json.go
-
-	type foo struct {
-		Field        string
-		SpecialField string
-	}
-
-	type fooMarshaling struct {
-		SpecialField specialString // overrides type of SpecialField when marshaling/unmarshaling
-	}
+See [the documentation for more details](https://godoc.org/github.com/fjl/gencodec).