doc(resolveConfig): document options.config (#3050)

README.md

@@ -470,8 +470,10 @@ prettier.formatWithCursor(" 1", { cursorOffset: 2 });
 
 #### `prettier.resolveConfig(filePath [, options])`
 
-`resolveConfig` can be used to resolve configuration for a given source file.
-The function requires an input file path as an argument, you might want to pass `process.cwd()` for searching from the current working directory.
+`resolveConfig` can be used to resolve configuration for a given source file, passing its path as the first argument.
+The config search will start at the file path and continue to search up the directory (you can use `process.cwd()` to start
+searching from the current directory).
+Or you can pass directly the path of the config file as `options.config` if you don't wish to search for it.
 A promise is returned which will resolve to:
 * An options object, providing a [config file](#configuration-file) was found.
 * `null`, if no file was found.