Add PPA instructions for Ubuntu
Add PPA instructions for Ubuntu

Since Debian and Ubuntu "stable" or LTS releases are typically far behind the Git team supported releases.
mosoft521/gitscm-next,git/git-scm.com,jasonlong/git-scm.com,Mokolea/git-scm.com,Mokolea/git-scm.com,mosoft521/gitscm-next,jasonlong/git-scm.com,Mokolea/git-scm.com,Mokolea/git-scm.com,jasonlong/git-scm.com,git/git-scm.com,git/git-scm.com,mosoft521/gitscm-next,mosoft521/gitscm-next,git/git-scm.com,jasonlong/git-scm.com