Add screencast on writing readmes

[janosg]

We should add a screencast on writing readmes in final projects. The corresponding grading guidelines are:

• Does the readme file provide a good entry point to the project?
• Does it contain everything you need to get the project to run (assuming you do not
  know the project templates)
• Does formatting and language look like they cared?
• You can also deduct if it is too long