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