docs: add critical setup rules for local development and debugging

Added two new sections to README:

1. "Local Development / Manual Install" section:
   - Shows correct pattern for loading xterm-color in :preface
   - Explains :demand t ensures immediate loading
   - Provides both use-package and manual install examples
   - Emphasizes xterm-color must load BEFORE wttrin

2. "Debugging and Troubleshooting" section:
   - Documents wttrin-debug must be set BEFORE loading wttrin
   - Shows correct vs incorrect examples (with ❌ for wrong way)
   - Explains how to view debug log with M-x wttrin--debug-show-log
   - Includes example debug output
   - Lists common issues and solutions

Key rules documented:
- xterm-color in :preface with :demand t for local development
- wttrin-debug set before (require 'wttrin) or use-package declaration
- Debug checked at load time, can't be set in :custom section

These are common pitfalls that prevented wttrin from loading correctly.

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Co-Authored-By: Claude <noreply@anthropic.com>

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