Add documentation and tests for Either (#533)

How do I regenerate the documentation?

---------

Co-authored-by: Li Haoyi <[email protected]>

Author: jberkel

upickle/test/src/upickle/example/ExampleTests.scala

@@ -208,6 +208,12 @@ object ExampleTests extends TestSuite {
         write(Some(1))                    ==> "1"
         write(None)                       ==> "null"
       }
+
+      test("either") {
+        write(Left(42): Either[Int, String])     ==> "[0,42]"
+        write(Right("foo"): Either[Int, String]) ==> """[1,"foo"]"""
+      }
+
       test("tuples"){
         write((1, "omg"))                 ==> """[1,"omg"]"""
         write((1, "omg", true))           ==> """[1,"omg",true]"""

upickleReadme/Readme.scalatex

@@ -152,6 +152,12 @@
 
         @hl.ref(exampleTests, Seq("\"more\"", "\"options\"", ""))
 
+        @p
+          @code{Either}s are serialized as JSON lists with 2 elements, with the first
+          element a number indicating @code{Left} (0) or @code{Right} (1), and the second
+          containing the value.
+
+        @hl.ref(exampleTests, Seq("\"more\"", "\"either\"", ""))
 
         @p
           Tuples of all sizes (1-22) are serialized as heterogenous JSON lists