Improve installatoin instructions.

Specifically refer to pip3 instead of pip to avoid potential
confusion.

Also mention virtualenv

docs/global.rst.inc

@@ -23,3 +23,4 @@
 .. _Arch Linux: https://aur.archlinux.org/packages/attic/
 .. _Slackware: http://slackbuilds.org/result/?search=Attic
 .. _Cython: http://cython.org/
+.. _virtualenv: https://pypi.python.org/pypi/virtualenv/

docs/index.rst

@@ -32,7 +32,7 @@ Easy installation
 -----------------
 You can use pip to install |project_name| quickly and easily::
 
-    $ pip install attic
+    $ pip3 install attic
 
 |project_name| is also part of the Debian_, Ubuntu_, `Arch Linux`_ and Slackware_
 distributions of GNU/Linux.

docs/installation.rst

@@ -19,11 +19,14 @@ Newer versions are available from homebrew_ on OS X and from FreeBSD ports.
 The llfuse_ python package is also required if you wish to mount an
 archive as a FUSE filesystem.
 
+Virtualenv_ can be used to build and install |project_name|
+without affecting the system Python or requiring root access.
+
 Installing from PyPI using pip
 ------------------------------
 ::
 
-    $ pip install Attic
+    $ pip3 install Attic
 
 Installing from source tarballs
 -------------------------------