Add equ explanation

Assembler-directives.md

@@ -1,4 +1,24 @@
-TODO:
+# equ
+
+The `equ` directive allows you to give a name to certain special values. This code:
+
+    INT_LEGS equ 0x0001
+    LEGS_SET_DIRECTION equ 0x0001
+    NORTH equ 0x0000
+
+    MOV A, LEGS_SET_DIRECTION
+    MOV B, NORTH
+    HWI INT_LEGS
+
+translates to:
+    
+    MOV A, 0x0001
+    MOV B, 0x0000
+    HWI 0x0001
+
+but is much easier to understand.
+
+# TODO:
 labels  
 equ  
 dup