docs: add usage section to README

Author: drlkf

README.md

@@ -4,6 +4,36 @@ A logger effect for the
 [`fused-effects`](https://github.com/fused-effects/fused-effects) library, based
 on the [`monad-logger`](https://github.com/snoyberg/monad-logger) API.
 
+# Usage
+In addition to `fused-effects-logger`, you most probably want to add
+`monad-logger` as well in order to use its logging functions.
+
+Here is a simple example to start running a program that logs to `stderr`:
+
+```haskell
+{-# LANGUAGE OverloadedStrings #-}
+{-# LANGUAGE TemplateHaskell #-}
+
+module Main (main) where
+
+import Control.Carrier.Lift (runM)
+import Control.Carrier.Logger.IO (runStderrLoggerIO)
+import Control.Effect.Lift (sendM)
+import Control.Monad.Logger (logInfo)
+
+main :: IO ()
+main = runM . runStderrLoggerIO $ do
+  $(logInfo) "hello from fused-effects-logger"
+  sendM (print 42)
+```
+
+It should produce the output:
+
+```
+[Info] hello from fused-effects-logger @(test-logger-0.1.0.0-KTuyx50rxPBAwC1EHp8UuV-test-logger-exe:Main app/Main.hs:13:4)
+42
+```
+
 # Contributing
 
 Please follow the process describe in [`RELEASE.md`](./RELEASE.md) to release