doc: improve primer

doc/primer.txt

@@ -149,13 +149,31 @@ with the upper bound. Example:
 
 Will print numbers 0 to 41, separated by a space.
 
-# Variables
+# Memory access and HERE
 
 We can read and write to arbitrary memory address with @ and !
 (C@ and C! for bytes). For example, "1234 0x8000 !" writes the
-word 1234 to address 0x8000.
+word 1234 to address 0x8000. We call the @ and ! actions
+"fetch" and "store".
 
-The word "VARIABLE" link a name to an address. For example,
+There's a 3rd kind of memory-related action: "," (write). This
+action stores value on PS at where a special variable called
+"HERE" points to, and then advances HERE by 2 (there's also
+"C," for bytes).
+
+Note that the HERE word returns the address containing the
+pointer (it doesn't change). There's a shortcut word for
+"HERE @" named "H@", which is used much more often.
+
+HERE is initialized at the first writable address in RAM, often
+directly following the latest entry in the dictionary. Explain-
+ing the "culture of HERE" is beyond the scope of this primer,
+but know that it's a very important concept in Forth. For examp-
+le, new word definitions are written to HERE.
+
+# Variables
+
+The word "VARIABLE" links a name to an address. For example,
 "VARIABLE FOO" defines the word "FOO" and "reserves" 2 bytes of
 memory. Then, when FOO is executed, it pushes the address of the
 "reserved" area to PS.
@@ -163,6 +181,36 @@ memory. Then, when FOO is executed, it pushes the address of the
 For example, "1234 FOO !" writes 1234 to memory address reserved
 for FOO.
 
+Another way to create a variable is with the word CREATE, which
+creates a variable entry without reserving anything for it: it's
+your responsibility to reserve memory for it after you call it.
+It can be useful for arrays. For example, look at VARIABLE's
+definition:
+
+: VARIABLE CREATE 2 ALLOT ;
+
+# DOES>
+
+Calling DOES> makes the newly created entry into a special
+"does word" which behaves like a variable, that is, it pushes
+the address of the "reserved" space to PS, but with additional
+behavior attached to it.
+
+DOES> must be called in the context of a word definition and
+calling it stops the definition right there. Every word follow-
+ing the DOES> is our new entry's behavior. For example, let's
+look at CONSTANT's definition:
+
+: CONSTANT CREATE , DOES> @ ;
+
+A constant is created with "42 CONSTANT FOO" and FOO, instead
+of putting FOO's address on PS, puts 42 on it.
+
+You can see above that after we've created our FOO entry, we
+write it to HERE and then assign the behavior "@" to it, which
+means that it will transform the address currently on PS to its
+value.
+
 # IMMEDIATE
 
 We approach the end of our primer. So far, we've covered the