Recommend upgrading pip before Pillow, and use python -m pip

[hugovk]

Fixes #4204.

Changes proposed in this pull request:

• To use Pillow's Python 3.8 wheels on Windows, pip >= 19.2 is needed
• Recommend upgrading pip before installing Pillow
• Recommend using python -m pip over pip
  • https://snarky.ca/why-you-should-use-python-m-pip/
• Also remove $ and > prompts, it's easier to copy and paste, especially multilines
• In the future, we may be able to specify a minimum pip version
  • Require minimum pip version? pypa/pip#4145