docs: translate tutorials/progressive.md to English version (#966)

docs/source/en/tutorials/progressive.md

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+title: Progressive Development
+---
+
+Egg provides both [Plugin](../advanced/plugin.md) and [Framework](../advanced/framework.md), and the former has two loading modes includes `path` and `package`. Then how should we choose?
+
+Step-by-step example will be provided to demonstrate how to start coding development progressively.
+
+Find detail codes on [eggjs/examples/progressive](https://github.com/eggjs/examples/tree/master/progressive).
+
+## Getting Started
+
+Assume we are writing a code to analyze UA to implement the function below:
+
+- `ctx.isAndroid`
+- `ctx.isIOS`
+
+You can easily write it down after previous tutorials, let's have a quick review:
+
+Codes refer to [step1](https://github.com/eggjs/examples/tree/master/progressive/step1).
+
+Directory structure:
+
+```bash
+example-app
+├── app
+│   ├── extend
+│   │   └── context.js
+│   └── router.js
+├── test
+│   └── index.test.js
+└── package.json
+```
+
+Core code:
+
+```js
+// app/extend/context.js
+module.exports = {
+  get isIOS() {
+    const iosReg = /iphone|ipad|ipod/i;
+    return iosReg.test(this.get('user-agent'));
+  },
+};
+```
+
+## Prototype of Plugin
+
+Obviously, the logic is universal that can be written as a plugin.
+
+But since function might not perfect at the beginning, it might difficult to maintain if encapsulate into a plugin directly.
+
+We can put the code as the format of plugin, but not separate out.
+
+Codes refer to [step2](https://github.com/eggjs/examples/tree/master/progressive/step2).
+
+New directory structure:
+
+```bash
+example-app
+├── app
+│   └── router.js
+├── config
+│   └── plugin.js
+├── lib
+│   └── plugin
+│       └── egg-ua
+│           ├── app
+│           │   └── extend
+│           │       └── context.js
+│           └── package.json
+├── test
+│   └── index.test.js
+└── package.json
+```
+
+Core code:
+
+- `app/extend/context.js` move to `lib/plugin/egg-ua/app/extend/context.js`.
+
+- `lib/plugin/egg-ua/package.json` to declare plugin.
+
+```json
+{
+  "eggPlugin": {
+    "name": "ua"
+  }
+}
+```
+
+- `config/plugin.js` use `path` to mount the plugin.
+
+```js
+// config/plugin.js
+const path = require('path');
+exports.ua = {
+  enable: true,
+  path: path.join(__dirname, '../lib/plugin/egg-ua'),
+};
+```
+
+## Extraction to Independent Plugin
+
+The module's functions become more better after a period of developing so we could extract it out as an independent plugin.
+
+We extract an egg-ua plugin and have a quick review as below. Details refer to [Plugin](../advanced/plugin.md).
+
+Directory structure:
+
+```bash
+egg-ua
+├── app
+│   └── extend
+│       └── context.js
+├── test
+│   ├── fixtures
+│   │   └── test-app
+│   │       ├── app
+│   │       │   └── router.js
+│   │       └── package.json
+│   └── ua.test.js
+└── package.json
+```
+
+Codes refer to  [step3/egg-ua](https://github.com/eggjs/examples/tree/master/progressive/step3/egg-ua).
+
+Then modify the application, details refer to [step3/example-app](https://github.com/eggjs/examples/tree/master/progressive/step3/example-app).
+
+- Remove directory `lib/plugin/egg-ua`.
+- declare dependencies `egg-ua`  in `package.json`.
+- change type to `package` in `config/plugin.js`.
+
+```js
+// config/plugin.js
+exports.ua = {
+  enable: true,
+  package: 'egg-ua',
+};
+```
+
+**Note：We can use `npm link` for local test before releasing the plugin. Details refer to  [npm-link](https://docs.npmjs.com/cli/link).**
+
+```bash
+$ cd example-app
+$ npm link ../egg-ua
+$ npm i
+$ npm test
+```
+
+## Finally: A Framework
+
+After repeating the process above, we accumulate a few plugins and configurations, and might found that most of our team projects are using them.
+
+At that time, you can consider to abstract them as a framework which suitable for business scenarios.
+
+Firstly, abstract the example-framework as below. Let's have a quick review, details refer to [Framework](../advanced/framework.md).
+
+Directory structure:
+
+```bash
+example-framework
+├── config
+│   ├── config.default.js
+│   └── plugin.js
+├── lib
+│   ├── agent.js
+│   └── application.js
+├── test
+│   ├── fixtures
+│   │   └── test-app
+│   └── framework.test.j.
+├── README.md
+├── index.js
+└── package.json
+```
+
+- Codes refer to [example-framework](https://github.com/eggjs/examples/tree/master/progressive/step4/example-framework).
+- Remove the dependencies of plugins such as `egg-ua` and remove it from example-app, then configure them into the `package.json` and `config/plugin.js` of the framework.
+
+Then modify the application, details refer to [step4/example-app](https://github.com/eggjs/examples/tree/master/progressive/step4/example-app).
+
+- Remove declaring dependencies `egg-ua` in `config/plugin.js`.
+- Remove dependencies `egg-ua` in `package.json`.
+- declare dependencies `example-framework`  in `package.json` and configure the `egg.framework`.
+
+```json
+{
+  "name": "progressive",
+  "version": "1.0.0",
+  "private": true,
+  "egg": {
+    "framework": "example-framework"
+  },
+  "dependencies": {
+    "example-framework": "*"
+  }
+}
+```
+
+**Note：We can use `npm link` for local test before releasing the framework [npm-link](https://docs.npmjs.com/cli/link).**
+
+```bash
+$ cd example-app
+$ npm link ../egg-framework
+$ npm i
+$ npm test
+```
+
+## Last
+
+In conclusion, we can see how to make the framework evolution step by step which benefits from Egg provides the powerful plugin mechanism, code co-build, reusability and modularity.
+
+
+- in general, put codes into `lib/plugin` if it can be reused in the application.
+- separate it into a `node module` when plugin becomes stable.
+- application with relatively reusable codes will work as a separate plugin.
+- abstract as framework to release after application become certain solutions of specified business scenario.
+- it would be a great improvement of teamwork after plugins extract, modularize and finally as a framework, beacuase other projects could reuse codes by just `npm install`.
+
+- **Note：Whether application/plugin/framework, unittest is a must and try to reach 100% coverage**