Installation instructions are outdated

[drittelhacker]

Since the PYPI package is frozen on version 1.1.1, which requires Python 2.7, the current Getting it running section in the README is not really helpful anymore. I see two options:

1. Update the PYPI package to the latest version
2. Actually rewrite the README to use pip install git+https://github.com/bootlin/pdf-link-checker.git

The latter saved my day, when digging it out of #3.

Besides this, many thanks for bringing this helpful tool to Python 3, with the new versions.

[michaelopdenacker]

Thanks for the suggestion.
I'm taking option #2.
Fixed by ce9129a
Thanks again
Michael