docs: update README for alternative config path (#419)

README.md

@@ -55,10 +55,18 @@ See `config.example.toml` for an example configuration file.
 
 ### Configuration Path
 
-The configuration should be placed in the following paths depending on the operating system:
+#### `CONFIG_DIR` on each platform
+- **Windows**: `%APPDATA%`
+- **macOS** and **other Unix systems**: `${XDG_CONFIG_HOME:-~/.config}`
 
-- **Windows** - `%APPDATA%/topgrade.toml`
+`topgrade` will look for the configuration file in the following places, in order of priority:
-- **macOS** and **other Unix systems** - `${XDG_CONFIG_HOME:-~/.config}/topgrade.toml`
+
+1. `CONFIG_DIR/topgrade.toml`
+2. `CONFIG_DIR/topgrade/topgrade.toml`
+
+If the file with higher priority is present, no matter it is valid or not, the other configuration files will be ignored.
+
+On the first run(no configuration file exists), `topgrade` will create a configuration file at `CONFIG_DIR/topgrade.toml` for you.
 
 ### Custom Commands