Add README file for CatalogUrlRewrite, update README file for Developer and Csp module

Author: ravi-chandra3197

app/code/Magento/CatalogUrlRewrite/README.md

@@ -0,0 +1,11 @@
+# Magento_CatalogUrlRewrite module
+
+This module generate url rewrite fields for catalog and product. 
+
+## Extensibility
+
+Extension developers can interact with the Magento_CatalogUrlRewrite module. For more information about the Magento extension mechanism, see [Magento plugins](https://devdocs.magento.com/guides/v2.4/extension-dev-guide/plugins.html).
+
+[The Magento dependency injection mechanism](https://devdocs.magento.com/guides/v2.4/extension-dev-guide/depend-inj.html) enables you to override the functionality of the Magento_CatalogUrlRewrite module.
+
+A lot of functionality in the module is on JavaScript, use [mixins](https://devdocs.magento.com/guides/v2.4/javascript-dev-guide/javascript/js_mixins.html) to extend it.

app/code/Magento/Csp/README.md

@@ -1,2 +1,11 @@
+# Magento_Csp module
 Magento_Csp implements Content Security Policies for Magento. Allows CSP configuration for Merchants,
 provides a way for extension and theme developers to configure CSP headers for their extensions.
+
+## Extensibility
+
+Extension developers can interact with the Magento_Csp module. For more information about the Magento extension mechanism, see [Magento plugins](https://devdocs.magento.com/guides/v2.4/extension-dev-guide/plugins.html).
+
+[The Magento dependency injection mechanism](https://devdocs.magento.com/guides/v2.4/extension-dev-guide/depend-inj.html) enables you to override the functionality of the Magento_Csp module.
+
+A lot of functionality in the module is on JavaScript, use [mixins](https://devdocs.magento.com/guides/v2.4/javascript-dev-guide/javascript/js_mixins.html) to extend it.

app/code/Magento/Developer/README.md

@@ -1 +1,11 @@
+# Magento_Developer module
+
 The Magento_Developer module provides functionality to make it easier to develop in Magento 2.
+
+## Extensibility
+
+Extension developers can interact with the Magento_Developer module. For more information about the Magento extension mechanism, see [Magento plugins](https://devdocs.magento.com/guides/v2.4/extension-dev-guide/plugins.html).
+
+[The Magento dependency injection mechanism](https://devdocs.magento.com/guides/v2.4/extension-dev-guide/depend-inj.html) enables you to override the functionality of the Magento_Developer module.
+
+A lot of functionality in the module is on JavaScript, use [mixins](https://devdocs.magento.com/guides/v2.4/javascript-dev-guide/javascript/js_mixins.html) to extend it.