Create go-extension.md

Author: taowen

go-extension.md

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+---
+layout: default
+title: How to extend
+---
+
+
+# Bind-api extension
+
+Bind-api is the most complex api. Iterating the json input, while binding to a object graph has infinite ways to customize. 
+If the tool does not provide the flexibility to extend, then the user is forced to bind the object graph twice.
+The goal of jsoniter is not to be the fastest JSON parser, but to provide the flexibility I always wanted.
+The general direction to provide extensiblity, is to give the user a callback, instead of just a list of feature flags or annotation.
+All kinds of annotation support can be implemented using the callback. Also the callback should control the codegen if possible, not 
+slowing down the runtime execution.
+
+# Customize type decoding