"add Op name example and fix format error"

paddle/operators/name_convention.md

-## Operator Name Convention
+## Operator's Parameter Name Convention
 
 To make the operator document itself more clear, we recommend operator names obey the listing conventions.
 
-### OpMaker names
+### OpProtoMaker names
 
-When defining an operator in Paddle, a corresponding `OpMaker` need to be defined. All the `Input`/`Output` and `attrs` will write into the `OpProto` , and will be used in client language to create operator. 
+When defining an operator in Paddle, a corresponding [OpProtoMaker](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/framework/operator.h#L170) (TODO: OpProtoMaker Doc)need to be defined. All the Input/Output and Attributes will write into the [OpProto](https://github.com/PaddlePaddle/Paddle/blob/develop/paddle/framework/framework.proto#L61) , and will be used in client language to create operator. 
 
 - Input/Output.
-  -  names follow the `CamelCase` but the first character is uppercase. e.g. `X`, `Y`, `Matrix`, `LastAxisInMatrix`. Input/Output much more like Variables, we prefer to meaningful English words.
-  - If an operator's Input/Output are not meaningful words, input name starts from `X`. e.g. `X`, `Y`, and output name starts from `Out`. e.g. `Out`.
+  - Input/Output names follow the **CamelCase**. e.g. `X`, `Y`, `Matrix`, `LastAxisInMatrix`. Input/Output much more like Variables, we prefer to meaningful English words. 
+  - If an operator's Input/Output are tensors in math, not match to any meaningful words, input name should starts from `X`. e.g. `X`, `Y`, and output name should starts from `Out`. e.g. `Out`. This rule make operators which have few inputs/outputs unified.
 
-* Attribute.
-  * Attribute name follows the normal `CamelCase`. e.g. `x`, `y`, `axis`, `rowwiseMatrix`. Also, attribute name prefers to meaningful English words.
-* Comments.
-  * Input/Output/Attr comment follow the format of `type:meaning`. e.g. `AddOutput("Out", "EigenTensor,Tensor: Output of XX")`. we prefer to more meaningful comment. Some comments like `The first input of Operator` contains no information, we forbid it.
-  * Operator comment format of` R"DOC(your comment here)DOC"`. if there is math calculation in this operator, you should write the equation in the comment. e.g. `Out = X + Y`. 
+- Attribute.
+  - Attribute name follows the **camelCase**. e.g. `x`, `y`, `axis`, `rowwiseMatrix`. Also, attribute name prefers to meaningful English words.
+
+- Comments.
+  - Input/Output/Attr comment follow the format of **(type,default value) usage**, corresponding to which type it can be and how it will be used in the operator. e.g.  Attribute in Accumulator`"gamma" `,`(float, default 1.0) Accumulation multiplier`
+  - Operator comment format of` R"DOC(your comment here)DOC"`. You should explain the input/output of the operator first. If there is math calculation in this operator, you should write the equation in the comment. e.g. `Out = X + Y`. 
+
+- Order.
+  - Follow the order of Input/Output, then Attribute, then Comments. See the example in best practice.
 
 ### Best Practice
 
+Here we give some examples to show how these rules will be used.
+
 - The operator has one input, one output. e.g.`relu`, inputs: `X`, outputs: `Out`. 
 
 - The operator has two input, one output. e.g. `rowwise_add`, inputs : `X`, `Y`, outputs : `Out`.
 
 - The operator contains attribute. e.g. `cosine`, inputs : `X`, `axis`, outputs : `Out`.
 
-  ​
+  We give a full example of Accumulator Operator. Its OpProtoMaker should look like below.
+
+```c++
+class AccumulateOpMaker : public framework::OpProtoAndCheckerMaker {
+public:
+  AccumulateOpMaker(framework::OpProto *proto,
+                            framework::OpAttrChecker *op_checker)
+    : OpProtoAndCheckerMaker(proto, op_checker) {
+    AddInput("X", "(Tensor) The input tensor that has to be accumulated to the output tensor. If the output size is not the same as input size, the output tensor is first reshaped and initialized to zero, and only then, accumulation is done.");
+    AddOutput("Out", "(Tensor) Accumulated output tensor");
+    AddAttr<float>("gamma", "(float, default 1.0) Accumulation multiplier");
+    AddComment(R"DOC(
+Accumulate operator accumulates the input tensor to the output tensor. If the
+output tensor already has the right size, we add to it; otherwise, we first
+initialize the output tensor to all zeros, and then do accumulation. Any
+further calls to the operator, given that no one else fiddles with the output
+in the interim, will do simple accumulations.
+Accumulation is done as shown:
+
+Out = 1*X + gamma*Out
+
+where X is the input tensor, Y is the output tensor and gamma is the multiplier
+argument.
+)DOC");
+  }
+};
+```