Add ignore rules for Pipenv (#2977)

* Add ignore rules for Pipenv

Pipenv uses Pipfile.lock to maintain Python package information
(metadata, hash, etc.) installed as described in Pipfile. Thus,
Pipfile.lock may vary on different operating systems, platforms
when collaborating. This PR adds Pipfile.lock into the Python
default gitignore. See http://pipenv.org



* Update Python.gitignore

Not to ignore Pipfile.lock in default, but explain
when and why it should be ignored in case of
collaboration. (adjusted according to comment
in github/gitignore#2977 by @drothmaler )

* Apply suggestions from code review

Co-Authored-By: JarryShaw <jarryshaw@icloud.com>

* Update Python.gitignore

As suggested by @shiftkey , elaborate on the problems that users might see with `Pipfile.lock`.