Added class diagram

Author: fhessel

extras/README.md

@@ -2,6 +2,11 @@
 
 This folder contains files that are not directly part of the library.
 
+## Documentation
+
+The [docs](docs/) folder contains documentation about the internal structure
+of the library.
+
 ## Legacy folder
 
 Before the repository has been converted to follow the Arduino library

extras/docs/README.md

@@ -0,0 +1,11 @@
+# Documentation
+
+This folder contains documentation of the internal structure of the library.
+
+If not stated otherwise, diagrams have been created using [Umlet](https://www.umlet.com/).
+
+## Class diagram
+
+The following diagram shows the internal class structure of the library.
+
+![Class diagram of esp32_https_server](https://github.com/fhessel/esp32_https_server/blob/master/extras/docs/esp32_https_server_classes.png?raw=true)