Add installation instructions

These installation instructions obsolete the README generated by
backup/backup (which I'll modify in a later commit). These should be
fully working (barring test suite failures, which I'm already opening
bugs for), so if they don't, I'd like to hear about any problems.
Fix one of the instructions

Oops, this instruction wasn't quite right in its description.
Merge pull request #9 from Sophira/install-instructions

Add installation instructions
Add placeholder for var/maintain.d

This directory needs to exist for general user maintenance operations to work.
It also needs to be chmodded 777, so I'll add that to the install instructions
in the next commit.

Also adding an exception for the .placeholder file in .gitignore.
Add chmod to install instructions

/dreamhack/var/maintain.d needs to be able to be written to by everybody.
Merge pull request #11 from Sophira/add-maintain-dir

Add var/maintain.d directory