update readme and add screenshots

README.md

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 # powerbanks
- A Minetest mod that adds portable powerbanks that can be used to recharge technic tools
- 
- Dependencies: default, technic
+
+[![License](https://img.shields.io/badge/License-MIT-brightgreen.svg)](LICENSE)
+
+## Overview
+
+This mod adds portable powerbanks that can be used to recharge technic tools (including those from other mods).
+
+![Overview Screenshot](screenshots/overview.png?raw=true "Overview Screenshot") 
+
+## Powerbank versions
+
+Currently there are three different powerbanks:
+
+- #### Powerbank Mk1
+  
+  - Capacity: 300kEU
+  
+  - Charging slots: 1
+  
+  - Recipe:
+    ![Mk1 Recipe](screenshots/mk1_recipe.png?raw=true "Mk1 Recipe")
+
+- #### Powerbank Mk2
+  
+  - Capacity: 600kEU
+  
+  - Charging slots: 2
+  
+  - Recipe:
+    ![Mk2 Recipe](screenshots/mk2_recipe.png?raw=true "Mk2 Recipe")
+
+- #### Powerbank Mk3
+  
+  - Capacity: 1200kEU
+  
+  - Charging slots: 3
+  
+  - Recipe:
+    ![Mk3 Recipe](screenshots/mk3_recipe.png?raw=true "Mk3 Recipe")
+
+## 
+
+## Dependencies
+
+Currently this mod only depends on the [technic](https://github.com/minetest-mods/technic) mod.