add doc for hack tool

hack/README.md

@@ -0,0 +1,24 @@
+# Hack Go Runtime #
+
+This is a hack tool to copy const and type declarations from go source and inject hacked code.
+With this hack, it's possible to expose any interesting type to wild world.
+
+## Play with it ##
+
+Download a go src tar file from https://golang.org/dl/, extract all files to a folder and
+run following command.
+
+    ./hack.sh path-to-go-src
+
+## How it works ##
+
+The basic idea is quite simple: If we can get internal type declarations and make it public,
+then we can use it easily. So what I do is to parse all interesting go source files and copy
+types to a folder and inject some hacked code to make some types public.
+
+The `generator.go` is the key file to make it happen. It loads all hacks and walk through
+all AST tree nodes in a file to find out all constants and types. It works quite well with
+internal package and ignored files, as these features are required for go1.6 and greater.
+
+There is only 1 hack implemented in `runtime_hacker.go`. If we want to add more hacks, just
+implement more hacker files.