Improvements to main README file and reference documentation

[rsbrooks]

Made the following updates to the main README file (docs/intro.rst) to improve readability and add missing information:

• Changed 'Introduction' section header to 'What is wcwidth?'.
• Reformatted 'Introduction' section and fixed minor grammatical issues.
• Added 'Getting Started' section.
• 'Getting Started' section includes basic API function information, links to PyPI package, links to API documentation site, Unicode environment variable config, and pip install instructions
• Added 'Helpful Resources for Contributors' section.
• 'Helpful Resources for Contributors' section includes instructions for making local changes to source code, updating Unicode data tables, building documentation, executing unit tests, updating requirements, and using supplemental utilities.