Add running instructions

README.md

@@ -1,3 +1,25 @@
 # Advent of Code
 
 This code may contain spoilers, if you haven't finished the puzzles over on https://adventofcode.com.
+
+## Running
+
+Paste your input in a file named `input.txt` in the root of the project of the
+desired language. After that, follow the instructions for each language:
+
+### Rust
+
+```
+cargo run
+```
+
+### Elixir
+
+```
+$ iex
+iex(1)> c("main.ex")
+[DayX]
+iex(2)> DayX.load_file |> DayX.first
+YOUR ANSWER
+iex(3)>
+```