Docs/oauth quirks workarounds

[MattVAllen]

What

Added comprehensive documentation for E*TRADE OAuth integration with pyetrade, including known quirks, workarounds, and working code examples.

Why

The pyetrade library works with E*TRADE, but has several non-obvious quirks that cause issues for new users:

• Parameter names differ from E*TRADE docs (client_key vs consumer_key, etc.)
• Built-in OAuth flow doesn't work out-of-the-box with E*TRADE
• Token encoding and endpoint handling are undocumented
• API responses sometimes vary between list and dict formats

New users spend hours debugging these issues that could be avoided with clear documentation.

What's included

• docs/OAUTH_QUIRKS.md — Comprehensive guide covering:
  • Parameter name mapping
  • OAuth flow specifics and why the built-in flow fails
  • Token endpoint URLs and encoding requirements
  • API response variance and solutions
  • Complete working example code
• Updated README with link to guide