### What it does
Checks for string appends of the form `x = x + y` (without
`let`!).

### Why is this bad?
It's not really bad, but some people think that the
`.push_str(_)` method is more readable.

### Example
```
let mut x = "Hello".to_owned();
x = x + ", World";

// More readable
x += ", World";
x.push_str(", World");
```