Add tuple indexing exercise from @ruipserra!! 👏

Connects to #29

primitive_types/primitive_types6.rs

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+// Use a tuple index to access the second element of `numbers`.
+// You can put this right into the `println!` where the ??? is.
+// Scroll down for hints!
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+fn main() {
+    let numbers = (1, 2, 3);
+    println!("The second number is {}", ???);
+}
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+// While you could use a destructuring `let` for the tuple here, try 
+// indexing into it instead, as explained here:
+// http://doc.rust-lang.org/stable/book/primitive-types.html#tuple-indexing
+// Now you have another tool in your toolbox!