add blurb about += vs <<

git-svn-id: https://www.metasploit.com/svn/framework3/trunk@9745 4d416f70-5f16-0410-b530-b9f4589650da

HACKING

@@ -52,6 +52,23 @@ which always returns a String.  If you need the ASCII byte, unpack it like
 so: 
 	str[idx,1].unpack("C")
 
+6. Whenever possible, avoid using '+' or '+=' to concatenate strings.
+The '<<' operator is significantly faster. The difference will become
+even more apparent when doing string manipulation in a loop. The
+following table approximates the underlying implementation:
+	Ruby 		Pseudo-C
+	----------- 	----------------
+	a = b + c 	a = malloc(b.len+c.len+1);
+			strcpy(a, b);
+			memcpy(a+b.len, c, c.len);
+			a[b.len + c.len] = '\0';
+	a = b 		a = b;
+	a << c 		a = realloc(a, a.len+c.len+1);
+			memcpy(a+a.len, c, c.len);
+			a[a.len + c.len] = '\0';
+Note that the original value of 'b' is lost in the second case. Care
+must be taken to duplicate strings that you do not want to modify.
+
 
 Creating New Modules
 ====================