[Slim] Update README template

I'm not sure about `middlewareSrcPath` variable. I'll fix it in following PRs
if path is broken.
Hope that notice in README catches attention and most of users will read it.

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/PhpSlimServerCodegen.java

@@ -118,6 +118,9 @@ public class PhpSlimServerCodegen extends AbstractPhpCodegen {
 
         additionalProperties.put("middlewarePackage", middlewarePackage);
 
+        // make middleware src path available in mustache template
+        additionalProperties.put("middlewareSrcPath", "." + File.separator + toSrcPath(middlewarePackage, srcBasePath));
+
         supportingFiles.add(new SupportingFile("README.mustache", getPackagePath(), "README.md"));
         supportingFiles.add(new SupportingFile("composer.mustache", getPackagePath(), "composer.json"));
         supportingFiles.add(new SupportingFile("index.mustache", getPackagePath(), "index.php"));

modules/openapi-generator/src/main/resources/php-slim-server/README.mustache

@@ -7,4 +7,15 @@ is an example of building a PHP Slim server.
 
 This example uses the [Slim Framework](http://www.slimframework.com/).  To see how to make this your own, please take a look at the template here:
 
-[TEMPLATES](https://github.com/openapitools/openapi-generator/tree/master/modules/openapi-generator/src/main/resources/slim/)
+[TEMPLATES](https://github.com/openapitools/openapi-generator/tree/master/modules/openapi-generator/src/main/resources/php-slim-server/)
+
+{{#authMethods}}
+{{^hasMore}}
+## Authentication
+{{/hasMore}}
+{{/authMethods}}
+{{#authMethods}}
+{{#isBasic}}
+> Important! To make Basic Authentication work you need to implement `verifyCredentials` method in [AuthBasic]({{middlewareSrcPath}}\AuthBasic.php) class.
+{{/isBasic}}
+{{/authMethods}}