doc: update writer implementation help

writer/README.md

@@ -19,4 +19,49 @@ type Writer interface {
 ### Implementations
 
 - [x] [Standard output file writer](./standard/README.md)
-- [ ] [Terminal output writer](./terminal/README.md)
+- [x] [Terminal output writer](./terminal/README.md)
+
+### How to customize your own writer?
+
+As you can see, the writer is a simple interface, you can implement your own
+writer to fit your needs. The prerequisites are:
+
+- Already understand `Writer` interface usage.
+- Have understood about paint a picture with go (of course you can refer [standard](./standard) implementation).
+- QR Code matrix is a 2D array of `martix.State`, we can simply divide these states into
+  binary value (0/1), as the following table:
+
+|  State Expr  | value |                 representation                  |
+|:------------:|:-----:|:-----------------------------------------------:|
+|  StateFalse  |   0   |              unset (data and etc)               |
+|     ZERO     |   0   |              same as `StateFalse`               |
+|  StateTrue   |   1   |                   set (data)                    |
+|  StateInit   |   1   | not changed since initialized (temporary state) |
+| StateVersion |   1   |                set (qr version)                 |
+| StateFormat  |   1   |                 set (qr format)                 |
+| StateFinder  |   1   |                 set (qr finder)                 |
+
+Now, you can implement your own writer to fit your needs, let's use pseudocode to discuss: 
+
+```text
+// define your own writer structure to implement `Writer` interface.
+object writer {};
+
+writer.Write(matrix.Matrix):
+	// these should be `IMAGE` stream controller, it receives matrix
+	// it decide how to print the image. 
+	object paint;  
+	
+	// set use BLACK, unset use WHITE. Or provides matrix.State to colors mapping
+	// so that you can control QR Image output intensively. 
+	foreach row in matrix:
+		foreach column in row:
+			if column.State in [StateFalse]:
+				// paint a WHITE square block
+				paint.draw(x, y, WHITE);
+			else:
+				// paint a BLACK square block, 
+				paint.draw(x, y, BLACK);
+	// loop end;
+	// output paint;
+```