Resolve #3452289 "Add step by step documentation"

Update README with more specific steps to install, configure and test.

Closes #3452289

README.md

@@ -36,13 +36,51 @@ Download it from [CloudFront Cache Path Invalidate](https://www.drupal.org/proje
   `$ composer require 'drupal/cloudfront_cache_path_invalidate'`
 
 ## Configuration
-This needs the following settings:
-Please use below $settings with your AWS Credential in settings.php file
 
-$settings['aws.distributionid'] = '';
-$settings['aws.region'] = '';
-$settings['s3fs.access_key'] = '';
-$settings['s3fs.secret_key'] = '';
+To configure the module to invaliate desired paths in Cloudfront, we weill need to provide AWS credentials and configure which entity types to trigger invalidations.
+
+### 1. AWS credentials
+
+The module needs the following AWS credentials to be configured as part of the $settings array in settings.php or settings.local.php.
+
+
+```
+$settings['aws.distributionid'] = ''; // e.g. 'ABCD1234EFGH7'
+$settings['aws.region'] = ''; // e.g. 'eu-west-2'
+$settings['aws.access_key'] = ''; // e.g. 'AKIAIOSFODNN7EXAMPLE'
+$settings['aws.secret_key'] = ''; // e.g. 'wJalrXUtnFEMI/K7MDENG/bPxRfiCYEXAMPLEKEY'
+```
+
+You will need to generate these from your AWS Identity and Access Management (IAM) control panel. 
+
+For more information on managing IAM users and keys, see https://docs.aws.amazon.com/IAM/latest/UserGuide/id_credentials_access-keys.html
+
+### 2. Configuring behaviour
+
+Go to /admin/config/services/auto-cloudfront-cache-entities and select the entity type and bundle you want to trigger an invalidation. 
+
+For example, for a 'Article' content type:  
+
+ - For "Entity type" select "Content type"
+ - for "Entity Bundle" select "Article"
+ - Toggle "Detail page"
+
+If you also wish to invalidate additional paths when the "Page" content type is updated, you can add thess to the "Cloudfront URL to invalidate" field. For example, you might want a views listing at /articles to be invalidated when you update one of your articles.
+
+Click "Submit.
+
+### 3. Test expected behaviour 
+
+When you now edit and save a "Page' node, the module will attempt to connect to AWS Cloudfront and invalidate the canonical path and the path alias of the edited node.
+
+### 3. Check the databse logs
+
+After configuring the module, try editing and saving content to trigger your cache clearing rules. If successful, we should see something like this in the recent log messgaes: 
+
+```
+Cloudfront Auto Cache Flush on Update entity_type: node_type bundle_type: article bundle: node entity_id: 123 and paths /articles,/articles/my-latest-updated-article
+```
+
 
 ## Maintainers
 - Prashant Mishra - [PrashantMishra](https://www.drupal.org/u/prashant-mishra)
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