docs: explain how to use custom slash commands from the SDK

[MukundaKatta]

Why

#120 keeps coming back: people try /help or a custom /foo from
.claude/commands/ against the Python SDK, get nothing, and assume it's
unsupported. The SDK already supports both paths the CLI uses
(setting_sources + cwd, or plugins), but the README never says so,
so the only places it's documented are buried inside
examples/setting_sources.py and examples/plugin_example.py.

This is the smallest fix that closes the loop: a README section right
after Working Directory that shows the two minimal recipes and links to
the runnable examples.

What

• README: new Custom Slash Commands subsection under Basic Usage: query() with two snippets and a note about the None / []
setting_sources defaults.
• Points at the existing examples (setting_sources.py,
  plugin_example.py) so people can see the full flow without copy-paste.
• No code changes. No type changes. No new fields on
  ClaudeAgentOptions.

I deliberately did not add a new slash_commands field on
AgentDefinition / ClaudeAgentOptions: slash commands are loaded by
the CLI from the filesystem (.claude/commands/ or a plugin), and the
SDK already has the right knobs for both. Adding a parallel registration
API would just give people a second way to do the same thing.

Tested

• diff against main's README shows only the new section is added; no
  surrounding text is touched.
• The two snippets mirror what examples/setting_sources.py (the
  project_and_user case) and examples/plugin_example.py already do
  end-to-end, so the runnable proof is the existing examples.

Closes #120.