docs: fix missing git prefix for pip install
docs: fix missing git prefix for pip install

* Adds missing 'git+' prefix for pip install.

* Rewords slightly to clarify difference between installing
  from PyPI and GitHub.

Signed-off-by: Dan Michael O. Heggø <aaea6e4e41b7ee5e29f60a7baad5e79159a19706@gmail.com>

inveniosoftware/invenio,tiborsimko/invenio,inveniosoftware/invenio,tiborsimko/invenio