add a short document on stack comments

FossilOrigin-Name: a89d9d182a7f490a2e7503bc52c45abc66a2934514c57c70fe47ddf9bef0238f

doc/StackComments.md

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+Most words in RETRO have a stack comment. These look like:
+
+    (-)
+    (nn-n)
+
+As with all comments, a stack comment begins with ( and should end with 
+ a ). There are two parts to the comment. On the left side of the - is
+what the word *consumes*. On the right is what it *leaves*.
+
+RETRO uses a short notation, with one character per value taken or left.
+In general, the following symbols represent certain types of values.
+
+    b, n, m, o, x, y, z are generic numeric values           
+    s represents a string
+    v represents a variable
+    p, a represent pointers
+    q represents a quotation
+    d represents a dictionary header
+    f represents a `TRUE` or `FALSE` flag.
+
+In the case of something like:  (xyz-m)
+RETRO expects z to be on the top of the stack, with y below it and x below
+the y value. And after execution, a single value (m) will be left on the
+stack.
+
+Words with no stack effect have a comment of (-)