build: add .tool-versions. (#18)

* Add `.tool-versions`.

* Go 1.20.

Author: mpaulucci

.tool-versions

@@ -0,0 +1,3 @@
+elixir 1.15
+erlang 25.3
+golang 1.20

README.md

@@ -1,9 +1,7 @@
 # lambda_ethereum_consensus
 
 ## Prerequisites
-
-- Elixir and mix [installed](https://elixir-lang.org/install.html) (e.g. with asdf, or brew).
-
+- [asdf](https://github.com/asdf-vm/asdf), see `.tool-versions` for the required versions.
 ## Installing and running
 
 There are Makefile targets for these tasks.

mix.exs

@@ -5,7 +5,7 @@ defmodule LambdaEthereumConsensus.MixProject do
     [
       app: :lambda_ethereum_consensus,
       version: "0.1.0",
-      elixir: "~> 1.14",
+      elixir: "~> 1.15",
       start_permanent: Mix.env() == :prod,
       deps: deps()
     ]