Update for.md

I recently discovered the issue for myself, that you can write the following code:

for i in 10..1{
println!("Hello World");
} 

This code does nothing, but you dont get a compiler warning and nothing and this isnt described in the rust handbook either. 
I dont think I can be the only beginner stumbling over this, so theres a possible solution.  I dont know if the way I wrote it fits in with the writing style of the book, but just adjust it as needed :)

Author: Itshardtogiveyourselfaname

src/flow_control/for.md

@@ -46,6 +46,19 @@ fn main() {
 }
 ```
 
+Just remember that even though you can compile the code when a>b, the loop gets 
+never executed.
+```rust,editable
+for i in 10..1{
+println!("fizzbuzz");
+}
+```
+If you want to count down, you need to use .rev() instead
+```rust,editable
+for i in (1..10).rev(){
+println!("fizzbuzz");
+}
+```
 ## for and iterators
 
 The `for in` construct is able to interact with an `Iterator` in several ways.