Nice, you are on the right track. Watch o
So, log into your normal user.
Install the library to
/home/username/Documents/Arduino/libraries/. To accomplish this, I would download a zip file from the Github account and install the library via the zip option in the Arduino IDE.
If you already have the folder
/home/username/Documents/Arduino/libraries/Arduboy then I would check, is that you have the permissions set correctly.
username@home ~/Documents/Arduino/libraries/ $ ls -al
I bet you find everything is owned by root. to fix this do,
username@home ~/Documents/Arduino/libraries/ $ chown -r username:username Arduboy/*
Now try compiling again. If a user doesn't own a file or isn't part of a group with access to read or write, it will tell the user a file doesn't exist.
Also remember, if you install anything, you might have to log out and log back in in order to reload the environment variables.
Note running something as sudo will still run the process as root. The reason I mentioned applications in linux not running in root is not as a security warning, but to let you know that's now how any of them are designed, so if you are running something as root, it will probably go way wrong.