README: Define and describe the port Tier levels.

MicroPython has quite a few ports now (20 of them), but not all are in the
same stage of development.  This commit attempts to define port Tier levels
and assign a Tier to each of the existing ports.

The main aim here is to set expectations for the level of support and
development each port gets.  And also lower the bar of entry for new ports
so they can enter at a low Tier and gradually rise up to Tier 1.

See prior art here:
- https://peps.python.org/pep-0011/
- https://doc.rust-lang.org/rustc/target-tier-policy.html
- https://docs.zephyrproject.org/latest/project/release_process.html#hardware-support-tiers

Signed-off-by: Damien George <damien@micropython.org>
Signed-off-by: Angus Gratton <angus@redyak.com.au>

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