Add admin.md (#19)

* Update admin.md

* Update ADMIN.md

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Co-authored-by: Tomasz Smolarek <tomasz.smolarek@escolasoft.com>

ADMIN.md

@@ -1,3 +1,29 @@
 # Admin panel documentation
 
-Coming soon documentation on this package
+## Templates definition
+
+The administration panel is used to manage SMS templates.
+If the event does not have a default template defined, SMS sending will fail.
+
+Templates are defined in the "Templates" and "SMS" tabs.
+From the template list, you can create, edit or delete a template.
+
+![Template-SMS - template list](https://github.com/EscolaLMS/Templates-SMS/assets/59400506/66d3e94d-4591-42ff-9ab6-e87e556bf65f)
+
+When defining a template, we have variables to use. There are two types of variables global and event-defined.
+Global variables store general, system-related information, personalized variables store information directly related to the event.
+
+Variables use a convention, with the @ sign before the variable name, to use a variable in a template you need to put the @ sign and the variable name, e.g. @VarSimpleName.
+The template must be marked as default for SMS sending to work.
+
+![Templates-SMS - template definition form](https://github.com/EscolaLMS/Templates-SMS/assets/59400506/de951e27-1c4d-4a29-a792-779980f3c3aa)
+
+
+## Configuration
+
+Configuration with the SMS dispatch platform is also required for proper operation.
+The connection can be configured in the Settings tab.
+Configuration example for Twilio platform:
+
+![Templates-SMS - configuration](https://github.com/EscolaLMS/Templates-SMS/assets/59400506/60c5b4fc-88a6-46ba-a9a4-93cedd98c5c3)
+