docs: mention that Erlang is a runtime dependency

close #75

manual/src/getting-started/installation.md

@@ -1,8 +1,11 @@
-# Installation
-
 Caramel works on macOS, Linux, and Windows. It is a single binary, and it has no
 external dependencies.
 
+The only prerequisite to run the code you compile with Caramel, is Erlang or
+Elixir. The [Elixir installation](https://elixir-lang.org/install.html) page
+lists many ways to install it comfortably, we suggest you to follow that step
+before continuing.
+
 ### Manual binary installation
 
 You can manuall install Caramel as well on Windows, macOS, or Linux by downloading