docs: Indicate that class cannot be used in heex templates

Author: Étienne Lévesque

README.md

@@ -47,6 +47,10 @@ This allows you to use the `class` and `class_name` functions in the template an
 ```eex
   <h1 class="_2313dsc-title">Hello world</h1>
 ```
+
+**Please note that `class` cannot be used in `heex` templates as the HTML
+validation engine does not allow it.**
+
 ## Advanced usage
 
 ### Phoenix.View

lib/ex_css_modules/ex_css_modules.ex

@@ -57,6 +57,10 @@ defmodule ExCSSModules do
 
   Returns nil for a class_name that does not exist.
 
+  Note that this function cannot be used with HEEX templates, as the HTML
+  validation engine does not allow them to be build with arbitrary text.
+  `class_name/3` should be used instead.
+
   ## Examples
 
       iex> class(%{ "hello" => "world"}, "hello")