docs(modify_letter): add description of the subject

subjects/modify_letter/README.md

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+## modify_letter
+
+### Instructions
+
+Create a **function** `remove_letter_sensitive` that returns a string without the letter specified as argument
+
+Create a **function** `remove_letter_insensitive` that returns a string without the letter specified as argument (ignoring case)
+
+Create a **function** `swap_letter_case` that returns a string swapping the case for the chosen letter.
+
+### Expected Functions
+
+```rust
+pub fn remove_letter_sensitive(s: &str, letter: char) -> String {
+}
+
+pub fn remove_letter_insensitive(s: &str, letter: char) -> String {
+}
+
+pub fn swap_letter_case(s: &str, letter: char) -> String {
+}
+```
+
+### Usage
+
+Here is a program to test your functions.
+
+```rust
+use modify_letter::*modify_letter*;
+
+fn main() {
+    println!("{}", remove_letter_sensitive("Joje jis mijssjing", "j"));
+    println!("{}", remove_letter_insensitive("jaillA ais swiaAmmingA", "A"));
+    println!("{}", swap_letter_case("hEllo therE", "e"));
+}
+```
+
+And its output
+
+```console
+$ cargo run
+Joe is missing
+Jill is swimming
+Hello thERe
+$
+```