feat(markdown_to_html): new optional exercise for rust piscine

subjects/markdown_to_html/README.md

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+## markdown_to_html
+
+### Instructions
+
+Create a function which converts markdown into HTML.
+
+To easier the task you'll only have to account for the first three titles (`#`, `##`, `###`) and for `bold` and `italic` (`**` and `*`).
+
+Parsing and converting markdown can be very tricky and some edge cases can require a lot of time to solve, for this reason we will limit the tests to correct and simple inputs.
+
+No tests will be more complicated than the one showed in the usage example.
+
+### Expected Function
+
+```rust
+pub fn markdown_to_html(s: &str) -> String {
+}
+```
+
+### Usage
+
+Here is a possible program to test your function,
+
+```rust
+const EXAMPLE: &str = "# This is a nice converter
+
+ ## It handle *all* titles
+
+## From # to ### with no problems
+
+### With attention to details ;)
+###With attention to details ;)
+
+> Blockquotes are handled too
+>if well formatted of course
+
+And **bold** and *italics* of course work as well.
+
+****
+
+**bold on
+multiple
+lines
+also**";
+
+fn main() {
+    println!("{}", markdown_to_html(EXAMPLE));
+}
+```
+
+And its output:
+
+```console
+$ cargo run
+<h1>This is a nice converter</h1>
+
+ <h2>It handle <em>all</em> titles</h2>
+
+<h2>From # to ### with no problems</h2>
+
+<h3>With attention to details ;)</h3>
+###With attention to details ;)
+
+<blockquote>Blockquotes are handled too</blockquote>
+>if well formatted of course
+
+And <strong>bold</strong> and <em>italics</em> of course work as well.
+
+<strong></strong>
+
+<strong>bold on
+multiple
+lines
+also</strong>
+$
+```