用大学python的水平帮我定义一个ReadFile函数，实现文本文件的读取

def ReadFile(filename):
"""
This function reads the contents of a text file and returns them as a string.

Parameters:
filename (str): The filename (including the path if it is not in the current directory) of the file to be read.

Returns:
A string containing the contents of the file.
"""
with open(filename, 'r') as f:
file_contents = f.read()
return file_contents