Remove reference to deprecated rule in README

[martis42]

PR Checklist

Please check if your PR fulfills the following requirements:

• Does not include precompiled binaries, eg. .par files. See CONTRIBUTING.md for info
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature (please, look at the "Scope of the project" section in the README.md file)
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

What is the current behavior?

When following the guidelines in the README for setting up a workspace using the pip packaging rules one receives the following warning when executing bazel commands:
DEBUG: .../external/rules_python/python/pip.bzl:61:10: DEPRECATED: the pip_repositories rule has been replaced with pip_install, please see rules_python 0.1 release notes
Caused here.

Issue Number: N/A

What is the new behavior?

By not including the depecated rule in the WORKSPACE file, no more warning is generated. No functionality is lost.

Does this PR introduce a breaking change?

• Yes
• No

Other information

N/A

[thundergolfer]

https://github.com/bazelbuild/rules_python/blob/master/docs/pip.md#pip_repositories is still around, though looks like the whole file needs a refresh, so can be done separately to this.

[martis42]

work was duplicated here (strangely after positiv review in this PR) #407

[thundergolfer]

Sorry @martis42, I think I neglected to merge it thinking that contributors had merge-after-approve abilities.