Initial config docs

Configuration-Files.md

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+Some itch-dl settings can be saved permanently. The configuration file lives in:
+
+- Windows: `C:/Users/<username>/AppData/Roaming/itch-dl/config.json`
+- macOS: `~/Library/Application Support/itch-dl/config.json`
+- Linux: `~/.config/itch-dl/config.json` (respects `XDG_CONFIG_HOME` if set)
+
+The `config.json` file is a JSON file with the following structure (make sure your file does **NOT** have comments):
+
+```json
+{
+    // itch.io API key
+    "api_key": "UeqKR3nBrVCxM32qt....Ga0FH2TwwbDPIDhseFZ"
+}
+```
+
+## Profiles
+
+Profiles are an easy way to switch between multiple configs - for example, if you have multiple accounts (main/developer/~~horny~~) and you'd like to easily switch the API key used for downloading a given title. By default, no profile is loaded.
+
+- Create a `profiles` directory next to the `config.json` file.
+- Create `<name>.json` files within that `profiles` directory. Their format is the same as the main `config.json` file.
+- When running itch-dl, use the `--profile <name>` argument to load a specified profile.
+
+The main `config.json` file is always loaded first, if it exists - the profile overrides values from the main config file.