README: Use 'pip install --user'

This is the preferred way of installing packages these days.

Signed-off-by: Stephen Finucane <stephen@that.guru>

Author: stephenfin

README.rst

@@ -63,7 +63,7 @@ Once dependencies are installed, clone this repo and run ``setup.py``:
 
    $ git clone https://github.com/getpatchwork/git-pw
    $ cd git-pw
-   $ sudo python setup.py
+   $ pip install --user .  # or 'sudo python setup.py'
 
 Getting Started
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