Make a general framework for doc in how_to_use_capi.

[peterzhang2029]

resolve #372

[hedaoyuan]

赞这个文档。
在Mobile上用户同样遇到how_to_use_capi的问题 See Inference API。另外，我有几个问题：

1. 这个PR还在开发吗？还是只差Review了？我可以来Review。
2. 这个文档可以移到Paddle Repo中吗？我觉得直接放到capi那个目录里面可能比较好。
3. 建议可以加一个图，解释整个编程模型的步骤，包括序列化模型->初始化->create gradient machine -> 组织输入数据 -> 前向计算 -> 获取数据 -> 清理。
4. 可以有独立的章节解释多线程计算，Batch Computing等。

[lcy-seso]

@hedaoyuan 这个PR会继续开发，README 我来继续写完。最近就搞定。

[wangkuiyi]

We will need English documentation. It could be in this PR, or a future PR.

For some old PRs in the PaddlePaddle/mobile repo, it was me who added the English document. It would be better if the original author can give an English draft, and I can help polish them.

[lcy-seso]

@wangkuiyi We will have a discussion with @hedaoyuan and @qingqing01 in this week to better organize this documentation and finally move it into the PaddlePaddle repo. We will add the English version documentation into the plan.

@hedaoyuan gave us some suggestions to re-organize the documentation to make it more informative by explaining the programming model and core concepts in PaddlePaddle rather than just copying our example codes. The outline and contents of this documentation will further be re-organized in the next PR.

[chengduoZH]

这里是否可以把每个选项的意思介绍一下，比如WITH_PYTHON=OFF为什么要关掉，WITH_AVX是该打开还是该关闭等