Update installation instruction in README

[amoghe]

On a pristine Ubuntu system, you also need libacl1-dev in order for the installation (via pip install) to succeed.

A future commit should also update the fact that you need some build tools (compiler, etc) setup, which can be installed on Ubuntu via the build-essential pkg.

[amoghe]

References issue #390

[amoghe]

This is a documentation change. Not sure why the checks should fail on this

[ThomasWaldmann]

@amoghe attic has not been updated since quite a while, because of that, tests are failing (always).

Have a look at borgbackup (fork based on Jonas' great work).

[amoghe]

thank you for the tip @ThomasWaldmann