DEV: clarify type map is reused

DEV: clarify type map is reused

diff --git a/README.md b/README.md
index 64473e6..a6a4675 100644
--- a/README.md
+++ b/README.md
@@ -130,10 +130,12 @@ When using Postgres, native type mapping implementation is used. This is roughly
 implemented as:
 
 `‍``ruby
-type_map = PG::BasicTypeMapForResults.new(conn)
+type_map ||= PG::BasicTypeMapForResults.new(conn)
 # additional specific decoders
 `‍``
 
+The type mapper instansitated once on-demand at boot and reused by all mini_sql connections.
+
 Initializing the basic type map for Postgres can be a costly operation. You may
 wish to amend the type mapper so for example you only return strings:
 

GitHub sha: 7d896b5f