doc: add roll 4d6 keep 3 lowest example

01.workspace/oxidice_lib/examples/roll_4d6_keep_3_lowest.rs

@@ -0,0 +1,10 @@
+use oxidice_lib::dice::*;
+
+fn main() {
+    let result = Handful::grab(4, 6).roll();
+    print!("{result} -> ");
+    let result = result.keep_lowest(3).unwrap();
+    print!("{result} -> ");
+    let result = result.sum().unwrap();
+    println!("{result}");
+}

01.workspace/oxidice_lib/src/imp/mod.rs

@@ -2,6 +2,7 @@ pub mod die_roll;
 pub mod handful_grab;
 pub mod handful_roll;
 pub mod outcome_keep_highest;
+pub mod outcome_keep_lowest;
 pub mod outcome_max;
 pub mod outcome_min;
 pub mod outcome_sum;

01.workspace/oxidice_lib/src/imp/outcome_keep_lowest.rs

@@ -0,0 +1,21 @@
+use crate::*;
+
+impl Outcome {
+    pub fn keep_lowest(self, dice_num: u16) -> Result<Outcome, FailedTo> {
+        if dice_num == 0 {
+            return Err(FailedTo::ProcessInput);
+        }
+        match self {
+            Outcome::Scalar(value) => Ok(Outcome::List(vec![value])),
+            Outcome::List(mut values) => {
+                values.sort();
+                Ok(Outcome::List(
+                    values.into_iter().take(dice_num as usize).collect(),
+                ))
+            }
+        }
+    }
+}
+
+// #[cfg(test)]
+// mod unit_tests { use super::*; }