new bridge setup guide: Arch Linux

by merging this request we:

• add Arch Linux to the list of "Bridge Relay Guides";
• setup obfs4proxy using AUR;
  • mention about how and why use AUR;
  • offer a link so people can opt to install that from source;
• keep up with previous efforts of adding consistency to all relay setup guides;
• let the key: empty for now.

https://github.com/torproject/community/pull/163