Updates README.md to include alternative installation and improved solution for HAML/Ruby conflict

Basically, I've just added a few lines to the README.md on a "Git-first" install method for anyone who doesn't have the Package Manager installed. Someone with Windows and Mac OS knowledge can make it mac friendly if they want.

Finally, the alternative method I've added is much simpler and also won't be overwritten by an update of Sublime Text.

Everything I've entered I've tested and it works...on Windows at least.