add calling convention notes

docs/call_conv.md

@@ -0,0 +1,10 @@
+# Calling convention
+## boot0
+I'm not sticking to any particular calling convention for boot0, since we have to golf to fit in
+440 bytes. Whatever's most convenient.
+
+## boot1
+- Arguments: ax, cx, dx, stack
+- Return value: ax, cf for booleans
+- ax may be clobbered. All other registers are preserved
+- Callee cleanup (i.e. the function pops stack arguments when returning)