README.txt is a file in the "thestanleyparable/bin" folder of both The Stanley Parable and The Stanley Parable Demonstration. On the same folder there are 3 more files: client.dll (normal); server.dll and matchmaking.dll
On The Stanley Parable the text inside says: sup
And on The Stanley Parable Demonstration the text says: Hi Timmy!
This could be just a joke or a hidden developer message, but this file was added intentionally.