docs: clarify first-backup tracks only config, memory lands on next /remember

[fdaviddpt]

Closes #65.

The "Back up your memory" snippet ends with git add .gitignore config.json + push. A first-time user runs it, pushes, and finds no memory in the backup — slug dirs aren't tracked until the after_save hook fires after the next /remember.

Added a note after the snippet covering all three asks from #65:

• initial commit only tracks .gitignore / config.json
• per-project slug dirs land automatically via the after_save hook on next /remember
• verify via cd ~/.remember && git log
• plus the optional git add <slug>/ for existing memory

Docs-only, 2 lines.