Improve structure in main.py, create requirements.txt and update README.

README.md

@@ -1,10 +1,33 @@
 # Serverpack-Downloader
-CLI Downloader to download server packs from curseforge
+CLI Downloader to download server packs from CurseForge
 
 # IMPORTANT
-add your own api key to the .env file
+Add your own CurseForge API-Key to an .env file
+```bash
+CURSEFORGE_API_KEY=YOUR_API_KEY
+```
+
+You can get your own through https://console.curseforge.com/
 
 # Dependencies
-dotenv python library \
-inquirer python library \
-requests python library
+```bash
+python-dotenv
+inquirer
+requests
+```
+
+Install with `pip install -r requirements.txt`
+
+
+# Notes
+
+If you get the following error, the Modpack you're trying to download most likely doesn't have a serverpack.
+
+```bash
+Traceback (most recent call last):
+  File "Serverpack-Downloader\main.py", line 90, in <module>
+    download_modpack(selected_modpack_name, selected_modpack_id)
+    ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  File "Serverpack-Downloader\main.py", line 64, in download_modpack
+    server_file_id = file['serverPackFileId']
+```