add more explanations of byte sequences

strings/488A489848

@@ -0,0 +1,15 @@
+Preserve registers on stack
+
+This is the standard form of saving the A X and Y registers on the stack
+
+PHA
+TXA
+PHA
+TYA
+PHA
+
+See, for example: 
+
+http://6502.org/tutorials/register_preservation.html
+
+Not copyrightable.