add documentation for plugins & refactor more

website/plugins.md

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+# Writing plugins for ImageVideoUpscaler
+
+Your plugin has to fulfill the following requirements:
+- Name of class it exposes has to be [name of engine]Scaler. The name of the file should be just the name of the engine in all lower case.
+- It has to support a folder or an individual file as an input and the function, has to take the following arguments:
+
+Argument | Variable type 
+---------|---------------
+x        | y
+
+- The function has to return the path to the folder where the final resulting images are located.