Reword deprecation message and add test to check that it warns

lib/erb_lint/cli.rb

@@ -217,7 +217,10 @@ def load_config
         config = RunnerConfig.new(file_loader.yaml(config_filename), file_loader)
         @config = RunnerConfig.default_for(config)
       elsif File.exist?(DEPRECATED_CONFIG_FILENAME)
-        warn(Rainbow("#{DEPRECATED_CONFIG_FILENAME} is deprecated. Rename your config to `erb_lint.yml`.").yellow)
+        deprecation_message = "The config file has been renamed to `#{DEFAULT_CONFIG_FILENAME}` and " \
+          "`#{DEPRECATED_CONFIG_FILENAME}` is deprecated. " \
+          "Please rename your config file to `#{DEFAULT_CONFIG_FILENAME}`."
+        warn(Rainbow(deprecation_message).yellow)
         config = RunnerConfig.new(file_loader.yaml(DEPRECATED_CONFIG_FILENAME), file_loader)
         @config = RunnerConfig.default_for(config)
       else

spec/erb_lint/cli_spec.rb

@@ -103,6 +103,22 @@ def run(processed_source)
       end
     end
 
+    context "with deprecated config file" do
+      let(:deprecated_config_filename) { ".erb-lint.yml" }
+      let(:config_file_content) { "---\nEnableDefaultLinters: true\n" }
+
+      before do
+        FileUtils.mkdir_p(File.dirname(deprecated_config_filename))
+        File.write(deprecated_config_filename, config_file_content)
+      end
+
+      it "shows a warning but loads the deprecated config file" do
+        expect { subject }.to(output(/`#{Regexp.escape(deprecated_config_filename)}` is deprecated/).to_stderr)
+        config = cli.instance_variable_get(:@config)
+        expect(config).to(be_an_instance_of(ERBLint::RunnerConfig))
+      end
+    end
+
     context "with --disable-inline-configs" do
       module ERBLint
         module Linters