Adds AUR link for Arch build

Adds URL to AUR ansile page
Adds suggestion to symlink python to python2 since most Arch users have
python3 by default. This suggestion might not be wise, though it doesn't
seem to break anything.
It might be probably wiser to have a patch for the AUR build that takes
care of this.

rst/gettingstarted.rst

@@ -94,6 +94,11 @@ Other Package Distributions
 
 Debian packages are in progress.
 
+`Arch PKGBUILD is available on AUR <https://aur.archlinux.org/packages.php?ID=58621>`_
+If you have python3 installed on Arch, you probably want to symlink python to python2.::
+
+    sudo ln -sf /usr/bin/python2 /usr/bin/python
+
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