add SetDecodedTimeAsUTC / SetDecodedTimeAsLocal; update README Announcement

[shamaton]

Summary (Breaking Change)

This PR changes the default Location used when decoding MessagePack Timestamp into Go’s time.Time from Local to UTC. The encoded/decoded instant is unchanged (MessagePack Timestamp stores seconds+nanos only); only time.Time.Location() differs.
Closes #51.

---

Motivation & Background

• MessagePack Timestamp represents an instant and does not carry timezone info.
• Decoding to Local by default leads to environment-dependent results (different hosts show different zones) and surprises in distributed systems/APIs.
• Defaulting to UTC provides a predictable, portable baseline.

---

What’s in this PR

• Default behavior (v3): Decoded time.Time is now UTC by default.

• Global switches (new):

  • msgpack.SetDecodedTimeAsUTC() – force decoded time.Time to use UTC (v3 default).
  • msgpack.SetDecodedTimeAsLocal() – force decoded time.Time to use Local (restores v2-like behavior).

• Docs:

  • Add a README Announcement near the top to highlight the change.

---

Public API (surface)

// Global configuration (process-wide). Call during init/main.
msgpack.SetDecodedTimeAsUTC()   // v3 default
// or
msgpack.SetDecodedTimeAsLocal() // restore v2-like behavior

• No change to the on-wire format or marshal behavior.
• The new functions are additive; the default change to UTC is the only breaking aspect.

---

Migration Guide (short)

• If you need the old behavior (Local):

  msgpack.SetDecodedTimeAsLocal()

• Recommended for distributed systems/APIs: keep or explicitly set the new UTC default:

  // Optional, v3 already defaults to UTC
  msgpack.SetDecodedTimeAsUTC()

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.18%. Comparing base (9d7a78e) to head (44fb0bc).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #52      +/-   ##
==========================================
+ Coverage   98.51%   99.18%   +0.67%     
==========================================
  Files          71       72       +1     
  Lines        6736     6766      +30     
==========================================
+ Hits         6636     6711      +75     
+ Misses         77       42      -35     
+ Partials       23       13      -10     

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