Add notes about how to do mkts "trimming"

Which is basically just "deleting" rows from a column series.
You can only use the trim command from the `.cmd` cli and only with a so
called `LocalClient` currently; it's also sketchy af and caused
a machine to hang due to mem usage..

Ideally we can patch in this functionality for use by the rpc api
and have it not hang like this XD

Pertains to https://github.com/alpacahq/marketstore/issues/264

piker/data/marketstore.py

@@ -230,8 +230,8 @@ _ohlcv_dt = [
     # ohlcv sampling
     ('Open', 'f4'),
     ('High', 'f4'),
-    ('Low', 'i8'),
-    ('Close', 'i8'),
+    ('Low', 'f4'),
+    ('Close', 'f4'),
     ('Volume', 'f4'),
 ]
 
@@ -547,6 +547,17 @@ class Storage:
                 if err:
                     raise MarketStoreError(err)
 
+    # XXX: currently the only way to do this is through the CLI:
+
+    # sudo ./marketstore connect --dir ~/.config/piker/data
+    # >> \show mnq.globex.20220617.ib/1Sec/OHLCV 2022-05-15
+    # and this seems to block and use up mem..
+    # >> \trim mnq.globex.20220617.ib/1Sec/OHLCV 2022-05-15
+
+    # relevant source code for this is here:
+    # https://github.com/alpacahq/marketstore/blob/master/cmd/connect/session/trim.go#L14
+    # def delete_range(self, start_dt, end_dt) -> None:
+    #     ...
 
 @acm
 async def open_storage_client(