Add compatibility note in docs

Author: rrrene

guides/introduction/installation.md

@@ -7,7 +7,7 @@ Add `:credo` as a dependency to your project's `mix.exs`:
 ```elixir
 defp deps do
   [
-    {:credo, "~> 1.6", only: [:dev, :test], runtime: false}
+    {:credo, "~> 1.7", only: [:dev, :test], runtime: false}
   ]
 end
 ```
@@ -17,3 +17,11 @@ And run:
 ```bash
 $ mix deps.get
 ```
+
+## Compatibility
+
+Credo aims to stay compatible with the list of [Elixir minor releases mentioned in the Elixir docs](https://hexdocs.pm/elixir/compatibility-and-deprecations.html).
+
+These are the releases that are actively tested on CI.
+
+Please note that Credo sometimes stays technically compatible with even earlier versions coincidentally.