Add deprecation warning for -keypath and -saltpath CLI args

Author: DarthSim

docs/configuration/options.mdx

@@ -22,6 +22,10 @@ You can also specify file paths using the command line by referencing a separate
 imgproxy -keypath /path/to/file/with/key -saltpath /path/to/file/with/salt
 ```
 
+:::warning
+`-keypath` and `-saltpath` command line arguments are deprecated and will be removed in imgproxy v4. Please use `IMGPROXY_KEY` and `IMGPROXY_SALT` environment variables instead.
+:::
+
 If you need a random key/salt pair really fast, as an example, you can quickly generate one using the following snippet:
 
 ```bash