Update documentation

docs/api/gb-programming-guide.md

@@ -0,0 +1,22 @@
+[< back to index](../index.md)
+
+# Game Boy programming guide
+
+**This guide is incomplete.
+Support for Game Boy targets is experimental and all information in this document may become obsolete.**
+
+## Program lifecycle
+
+The default Game Boy vectors are defined as following:
+
+The minimal Game Boy program thus looks like this:
+
+    void main() {
+        // initialize things
+        while(true) { }
+    }
+    
+    void on_vblank() {
+        // do things
+    }
+    

docs/api/target-platforms.md

@@ -78,6 +78,10 @@ The compiler emits tape images.
 
 * `zxspectrum_8080` – Sinclair ZX Spectrum 48k, using only Intel 8080 instructions
 
+* `gb_small` – a tiny 32K Game Boy program. (experimental)
+The compiler emits GB files.
+Read [the Game Boy programming guide](./gb-programming-guide.md) for more info.
+
 * `cpm` – CP/M on Intel 8080.
 The compiler emits COM files.