CoCo: add coco_crt target

docs/api/coco-programming-guide.md

@@ -2,6 +2,8 @@
 
 ### A note about Color Computer
 
+#### `coco_rsdos` target
+
 The `coco_rsdos` target creates binary files that can run on Tandy Color Computer running RS-DOS.
 
 The compiler output is a raw machine code file with the `.bin` extension, which then has to be put on a disk. 
@@ -13,3 +15,12 @@ You can do it using `imgtool` from the [MAME project](https://www.mamedev.org/):
 The resulting file can then be loaded and ran using the following commands:
 
     LOADM"CO":EXEC
+
+#### `coco_crt` target
+
+The `coco_crt` target creates 16k cartridge images that can run on Tandy Color Computer or Dragon.
+
+The program is run directly from ROM;
+typical ROM programming guidelines apply, see [the ROM vs RAM guide](./rom-vs-ram.md).
+
+The `main` function is not allowed to return.