`if no packages are given` sounds a bit better than
`if none is given` when referring to `provided packages`.
Also mention that `pipenv install` with no provided packages
installs all packages from Pipfile. It is not obvious for
people new to pipenv whether pipenv is using Pipfile or
Pipfile.lock by default.
Removed ppa mentions from documentation, and re-organised the page a bit
to make it structurally easier to upderstand.
Both the documentation's front page and README.rst now actively refer to
the page so user can be directed to better installation methods. (I
don't think this is very useful TBT because people don't use manuals,
but we should still do it.)
Also fixed many Sphinx warnings during documentation builds.
Pavel Sviderski
kennethreitz