Update documentation with sizefrom and boolean flags.

packing.go

@@ -57,19 +57,28 @@ following struct tag syntax is supported:
 
 Flags are comma-separated keys. The following are available:
 
-	type            A bare type name, e.g. int32 or []string.
+	type              A bare type name, e.g. int32 or []string.
 
-	sizeof=[Field]  Specifies that the field should be treated as a count of
-	                the number of elements in Field.
+	sizeof=[Field]    Specifies that the field should be treated as a count of
+					  the number of elements in Field.
 
-	skip=[Count]    Skips Count bytes before the field. You can use this to
-	                e.g. emulate C structure alignment.
+	sizefrom=[Field]  Specifies that the field should determine the number of
+	                  elements in itself by reading the counter in Field.
 
-	big,msb         Specifies big endian byte order. When applied to structs,
-	                this will apply to all fields under the struct.
+	skip=[Count]      Skips Count bytes before the field. You can use this to
+	                  e.g. emulate C structure alignment.
 
-	little,lsb      Specifies little endian byte order. When applied to structs,
-	                this will apply to all fields under the struct.
+	big,msb           Specifies big endian byte order. When applied to
+					  structs, this will apply to all fields under the struct.
+
+	little,lsb        Specifies little endian byte order. When applied to
+					  structs, this will apply to all fields under the struct.
+
+	variantbool       Specifies that the boolean `true` value should be
+	                  encoded as -1 instead of 1.
+
+	invertedbool      Specifies that the `true` and `false` encodings for
+					  boolean should be swapped.
 */
 func Unpack(data []byte, order binary.ByteOrder, v interface{}) (err error) {
 	defer func() {