docs(rust piscine): Add invert_sentence optional exercise subject

subjects/invert_sentence/README.md

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+## invert_sentence
+
+### Instructions
+
+Write a function called `invert_sentence` that takes a string as input and returns the words in the string in reverse order.
+In other words, the function should take a sentence as input and return a new sentence with the words reversed.
+
+```rust
+pub fn invert_string(string: &str) -> String {
+    // Your code goes here
+}
+```
+
+### Usage
+
+Here is a possible runner to test your function :
+
+```rust
+use invert_sentence::invert_sentence;
+
+fn main() {
+    println!("{}", invert_sentence("Rust is Awesome"));
+    println!("{}", invert_sentence("   word1     word2  "));
+    println!("{}", invert_sentence("Hello, World!"));
+}
+```
+
+And its output:
+
+```console
+$ cargo run | cat -e
+Awesome is Rust$
+word2 word1$
+World! Hello,$
+$
+```