Document undocumented SDK behaviour in the README - #60

- Listeners are called on the main thread (except tryIdentifyFromUrl's
  immediate error when no valid oeid is found).
- How the Google Advertising ID is read and attached, the 3 second wait,
  Limit Ad Tracking, that consent is the app's responsibility via
  skipAdvertisingIdDetection, and how to remove the merged AD_ID
  permission.
- Every OptableIdentifier's prefix and encoding, Hem/HashedPhoneNumber
  validation, and not passing hashed values to Email/PhoneNumber.
- The fields of OptableTargeting, and that profile returns one.

The Google Advertising ID text describes behaviour after the per-instance
GAID state and 3 second timeout fixes, so this should merge after them.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Match #62: every identify and targeting call reads the GAID (never
cached), waiting up to advertisingIdTimeoutMs (100 ms by default), and
skipAdvertisingIdDetection can be changed at any time, applying from the
next call.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Match #62: advertisingIdTimeoutMs is at least 1 ms, so zero no longer
means the GAID is not added, and reading the GAID typically takes tens
of milliseconds depending on the device. skipAdvertisingIdDetection is
the way to never add it.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>