Add 'sudo' in make install command

[adrianovogel]

The command 'make install' had to be run on my Ubuntu 18.04 as sudo: 'sudo make install'. Maybe it is a good idea to add it to documentation because the output error was not intuitively related to a lack of permission, like a 'permission denied'. The lack of permission was only a guess because one of the errors was "file INSTALL cannot copy file".