Further root readme bump, factor `.clearing` content

In line with our move to `uv` and recent `nix` support update a bunch of
the summary content and factor out the order-control section to a new
`.piker.clearing` readme file with embedded todos therein.

README.rst

@@ -22,6 +22,7 @@ we use much bleeding edge tech including (but not limited to):
 - `apache arrow and parquet`_ for time-series storage
 
 potential projects we might integrate with soon,
+
 - (already prototyped in ) techtonicdb_ for L2 book storage
 
 .. _comp_trader: https://jfaleiro.wordpress.com/2019/10/09/computational-trader/