Restore DOES> section in usage guide

I removed it because it was redundant with Starting Forth, but
I directly reference it in the dictionary, so well...

blk/003

@@ -12,5 +12,5 @@ Contents
 
  4 Number literals              6 Compilation vs meta-comp.
  8 Interpreter I/O             11 Signed-ness
-14 Addressed devices         
+14 Addressed devices           17 DOES>
 

blk/017

@@ -0,0 +1,16 @@
+DOES>
+
+Used inside a colon definition that itself uses CREATE, DOES>
+transforms that newly created word into a "does cell", that is,
+a regular cell ( when called, puts the cell's addr on PS), but
+right after that, it executes words that appear after the
+DOES>.
+
+"does cells" always allocate 4 bytes (2 for the cell, 2 for the
+DOES> link) and there is no need for ALLOT in colon definition.
+
+At compile time, colon definition stops processing words when
+reaching the DOES>.
+
+Example: ": CONSTANT CREATE HERE @ ! DOES> @ ;"
+

blk/040

@@ -8,7 +8,7 @@ CREATE x    --     Create cell named x. Doesn't allocate a PF.
 COMPILE x   --     Meta compiles. See B6.
 CONSTANT x  n --   Creates cell x that when called pushes its
                    value.
-DOES>       --     See B4.
+DOES>       --     See B17.
 IMMED?      a -- f Checks whether wordref at a is immediate.
 IMMEDIATE   --     Flag the latest defined word as immediate.
 LITA        n --   Write address n as a literal.