Add reference to howto repo (#19)

* Add reference to howto-foreign-generics repo

* add Example Usage section to README

Author: justinwoo

README.md

@@ -6,3 +6,37 @@ Generic deriving for `purescript-foreign`.
 
 - [Module Documentation](docs/Data/Foreign/Generic.md)
 - [Example](test/Main.purs)
+- [Further examples in this repo](https://github.com/justinwoo/purescript-howto-foreign-generic)
+
+## Example Usage
+
+```purescript
+import Data.Foreign.Class (class AsForeign, class IsForeign, readJSON, write)
+import Data.Foreign.Generic (defaultOptions, readGeneric, toForeignGeneric)
+import Data.Generic.Rep (class Generic)
+import Data.Generic.Rep.Show (genericShow)
+
+newtype MyRecord = MyRecord {a :: Int}
+
+derive instance genericMyRecord :: Generic MyRecord _
+
+instance isForeignMyRecord :: IsForeign MyRecord where
+  read = readGeneric $ defaultOptions {unwrapSingleConstructors = true}
+
+instance asForeignMyRecord :: AsForeign MyRecord where
+  write = toForeignGeneric $ defaultOptions {unwrapSingleConstructors = true}
+
+toJSONString = write >>> unsafeStringify
+fromJSONString = readJSON >>> runExcept
+
+main :: forall e. Eff (console :: CONSOLE | e) Unit
+main = do
+  log $ toJSONString (MyRecord {a: 1})
+  -- {a: 1}
+
+  log $ show eMyRecord
+  -- Right (MyRecord {a: 1})
+  where
+    eMyRecord :: Either _ MyRecord
+    eMyRecord = fromJSONString """{"a": 1}"""
+```