README: document release tags as recommended pinning method for downstream users

[rpcme]

Downstream users are pinning to arbitrary commit SHAs on release branches, which breaks when branch history is reconciled during ff-merge operations (see #15463).

We now publish tested release tags (e.g. whinlatter-2026.15.0, scarthgap-2026.15.0) for all active branches. The README should prominently document:

1. Release tags are the recommended way to pin meta-aws — they are stable, tested, and will not be affected by branch maintenance
2. Do not pin to commit SHAs on branches — branch history may change during routine ff-merge operations
3. Release cadence — weekly releases for active branches (master, scarthgap, whinlatter)
4. Where to find releases — link to https://github.com/aws4embeddedlinux/meta-aws/releases
5. Release naming convention — {branch}-{YYYY}.{WW}.{revision}

Example for downstream kas.yml or bblayers.conf:

repos:
  meta-aws:
    url: https://github.com/aws4embeddedlinux/meta-aws.git
    refspec: whinlatter-2026.15.0  # use release tag, not commit SHA

This was prompted by user reports in #15463 and via email from AGL users who were broken by branch reconciliation.