Add prerequisites for Arch Linux to the readme.
Merge pull request #3224 from duncankl/readme-arch-linux-setup

Add prerequisites for Arch Linux to the readme; r=Manishearth
Add a note about autoconf 2.13.

Many people seem to have this question.
Merge pull request #3225 from Ms2ger/note-autoconf

Add a note about autoconf 2.13; r=Manishearth