Adding MongoDB Transfer Howto

Setting-a-MongoDB-backend-to-use-as-database-provider-for-YTDL-M.md

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+Running YoutubeDL-Material with a MongoDB database backend will GREATLY increase stability,  
+performance and scalability for up to tens of thousands of video/audio downloads  
+(as of current testing, tell us if your dataset is larger and successful! :))
+
+Provided you have a MongoDB instance set up (either in a docker or even bare-metal) - you need to  
+put the connection string in **Settings > Database > MongoDB Connection String**
+
+    ![Screenshot 2022-03-25 125507](https://user-images.githubusercontent.com/1912133/160116383-687c2936-b91c-4571-9b49-afd39e072a87.png)
+
+1. _Change the IP and port_ to your MongoDB instance like shown above,
+2. click `Test connection string` and if it succeeds...
+3. hit `Save`
+4. and `Transfer Local to DB`.
+(mine says DB to Local because since I already set it up it offers me the other way around)
+
+That's it!
+
+YoutubeDL-Material will create the tables it needs on its own and fill them with the data from your Local DB.  
+**This process may take quite some time**, especially on large datasets, so be patient, please! You can use tools  
+like MongoDBCompass (MongoDB's official database inspector and editor) to check on the ingress of entries.