Add loopcounter section to the for-loop chapter

Sometimes loop counters are useful and we should show new users how it is achieved in Rust.

Author: azerupi

src/doc/trpl/for-loops.md

@@ -41,3 +41,38 @@ so our loop will print `0` through `9`, not `10`.
 Rust does not have the “C-style” `for` loop on purpose. Manually controlling
 each element of the loop is complicated and error prone, even for experienced C
 developers.
+
+# Loopcounter
+
+When you need to keep track of how many times you already looped, you can use the `.enumerate()` function.
+
+#### On ranges:
+
+```rust
+for (i,j) in (5..10).enumerate() {
+    println!("i = {} and j = {}", i, j);
+}
+```
+Outputs:
+```
+i = 0 and j = 5
+i = 1 and j = 6
+i = 2 and j = 7
+i = 3 and j = 8
+i = 4 and j = 9
+```
+Don't forget to add the parentheses around the range.
+
+#### On iterators: 
+```rust
+for (linenumber, line) in lines.enumerate() {
+    println!("{}: {}", linenumber, line);
+}
+```
+Outputs:
+```
+0: Content of line one
+1: Content of line two
+2: Content of line tree
+3: Content of line four
+```