Create issue_template.md

Change-Id: I5d31f9b8c794be4330f438b99f6227c15d2510f0

issue_template.md

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+### Expected behavior 期望表现
+{type something here...}
+
+
+### Actual behavior 实际表现
+{type something here...}
+
+
+### Steps to reproduce the problem 复现步骤
+1. {step 1}
+2. {step 2}
+3. {step 3}
+
+
+### Status of loaded data 数据状态
+
+#### Vertex/Edge summary 数据量
+- loaded vertices amount: {like 10 million}
+- loaded edges amount: {like 20 million}
+- loaded time: {like 200s}
+
+#### Vertex/Edge example 数据示例
+{type something here...}
+
+#### Schema(VertexLabel, EdgeLabel, IndexLabel) 元数据结构
+{type something here...}
+
+
+### Specifications of environment 环境信息
+- hugegraph version: {like v0.7.4}
+- operating system: {like centos 7.4, 32 CPUs, 64G RAM}
+- hugegraph backend: {like cassandra 3.10, cluster with 20 nodes, 3 x 1TB HDD disk each node}