shell version 3.20 added to metadata.json

Created by: brenoec

First pull request! If anything could be improved in the process, let me know.

I've encountered the same problems as @nos1609 following the steps at README.md file, but I was able to make it work with the tweak tool and a gnome session restart. After the command bellow nothing happened. I don't know if this is a problem.

gnome-shell-extension-tool -e hidetopbar@mathieu.bidon.ca

To get this working on gnome 3.20 I did these steps:

1. git clone https://github.com/mlutfy/hidetopbar/ (or my forked repo)
2. shell version 3.20 added to metadata.json
3. make schemas
4. zip repo folder
5. opened tweak tool and installed the .zip extension
6. restarted gnome session
7. enjoy it: life is good, thank you