Add acid-state to main README, include better example of usage.

README.md

@@ -17,6 +17,11 @@ to be paired up with two companion packages:
       `persistent`'s SQL backend.  Works with PostgreSQL, MySQL,
       SQLite.
 
+    * `serversession-backend-acid-state`: Storage backend using
+      `acid-state`.  This backend keeps sessions in memory but
+      provides ACID guarantees using a transaction log.  It can
+      also be used without durability as a memory-only backend.
+
   * _Frontend_, bindings for your web framework of choice.
     Currently we support:
 

serversession-backend-acid-state/src/Web/ServerSession/Backend/Acid.hs

@@ -4,12 +4,15 @@
 -- example:
 --
 -- @
--- import Data.Acid
--- import Web.ServerSession.Backend.Acid
+-- import Control.Exception (bracket)
+-- import Data.Acid.Local (openLocalState, createCheckpointAndClose)
+-- import Web.ServerSession.Backend.Acid (AcidStorage(..), emptyState)
 --
--- makeSessionStorage :: IO 'AcidStorage'
--- makeSessionStorage =
---   'AcidStorage' \<$\> openLocalState 'emptyState'
+-- withSessionStorage :: (AcidStorage -> IO a) -> IO a
+-- withSessionStorage =
+--   bracket
+--     ('AcidStorage' \<$\> openLocalState 'emptyState')
+--     createCheckpointAndClose
 -- @
 module Web.ServerSession.Backend.Acid
   ( AcidStorage(..)