Add missing file for concatenating strings

idioms/concat-format.md

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+# Concatenating strings with `format!`
+
+## Description
+
+It is possible to build up strings using the `push` and `push_str` methods on a
+mutable `String`, or using its `+` operator. However, it is often more
+convenient to use `format!`, especially where there is a mix of literal and
+non-literal strings.
+
+
+## Example
+
+```rust
+fn say_hello(name: &str) -> String {
+    // We could construct the result string manually.
+    // let mut result = "Hello";
+    // result.push_str(name);
+    // result.push('!');
+    // result
+
+    // But using format! is better.
+    format!("Hello {}!", name)
+}
+```
+
+
+## Advantages
+
+Using `format!` is usually the most succinct and readable way to combine strings.
+In nearly all cases it will generate optimal code in terms of performance too.
+