Improved installation instructions for Ubuntu
Improved installation instructions for Ubuntu

Added information on how to install from the repositories. Improved the instructions for how to build from source, for those that don't want an ancient version of Kakoune. On Ubuntu 20.04 and earlier, GCC 10 is not the default so we have to explicitly tell `make` to use GCC 10.

See also: https://github.com/mawww/kakoune/issues/4571

mawww/kakoune,mawww/kakoune,jkonecny12/kakoune,jkonecny12/kakoune,casimir/kakoune,jkonecny12/kakoune,casimir/kakoune,casimir/kakoune,casimir/kakoune,mawww/kakoune,mawww/kakoune,jkonecny12/kakoune