Document list_markets response shape in README

Signals that polyorderbooks is a published package on PyPI.

Co-authored-by: Cursor <cursoragent@cursor.com>

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The quickstart asked for point["b"] and point["a"]. The API returns
{t, bids, asks} per the OrderBookSnapshot schema, so those keys do not exist.
Both lookups used .get(...) or [], so there was no error — the snippet printed
"bid None ask None" while 15 bid and 26 ask levels sat in the response. A
developer trying the SDK would conclude the archive has no depth, on a product
whose whole point is full L2 depth.

Verified against the live API:

  before:  bid=None  ask=None   levels=0/0
  after:   bid=0.19  ask=0.2    levels=15/26

The README documented the markets response shape but never the books shape,
which is largely why the wrong keys went unnoticed. Adds it in the same style,
including that ladders are [price, size] best-first, that depth varies per
snapshot, and that the list can be empty when a window has no captures.

Read bids and asks from snapshots, and document the shape