add readme files to stage-N presets

Copied from the es2015/react presets.

packages/babel-preset-stage-0/README.md

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+# babel-preset-stage-0
+
+> Babel preset for stage 0 plugins.
+
+## Install
+
+```sh
+$ npm install --save-dev babel-preset-stage-0
+```
+
+## Usage
+
+### Via `.babelrc` (Recommended)
+
+**.babelrc**
+
+```json
+{
+  "presets": ["stage-0"]
+}
+```
+
+### Via CLI
+
+```sh
+$ babel script.js --preset stage-0
+```
+
+### Via Node API
+
+```javascript
+require("babel-core").transform("code", {
+  presets: ["stage-0"]
+});
+```

packages/babel-preset-stage-1/README.md

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+# babel-preset-stage-1
+
+> Babel preset for stage 1 plugins.
+
+## Install
+
+```sh
+$ npm install --save-dev babel-preset-stage-1
+```
+
+## Usage
+
+### Via `.babelrc` (Recommended)
+
+**.babelrc**
+
+```json
+{
+  "presets": ["stage-1"]
+}
+```
+
+### Via CLI
+
+```sh
+$ babel script.js --preset stage-1
+```
+
+### Via Node API
+
+```javascript
+require("babel-core").transform("code", {
+  presets: ["stage-1"]
+});
+```

packages/babel-preset-stage-2/README.md

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+# babel-preset-stage-2
+
+> Babel preset for stage 2 plugins.
+
+## Install
+
+```sh
+$ npm install --save-dev babel-preset-stage-2
+```
+
+## Usage
+
+### Via `.babelrc` (Recommended)
+
+**.babelrc**
+
+```json
+{
+  "presets": ["stage-2"]
+}
+```
+
+### Via CLI
+
+```sh
+$ babel script.js --preset stage-2
+```
+
+### Via Node API
+
+```javascript
+require("babel-core").transform("code", {
+  presets: ["stage-2"]
+});
+```

packages/babel-preset-stage-3/README.md

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+# babel-preset-stage-3
+
+> Babel preset for stage 3 plugins.
+
+## Install
+
+```sh
+$ npm install --save-dev babel-preset-stage-3
+```
+
+## Usage
+
+### Via `.babelrc` (Recommended)
+
+**.babelrc**
+
+```json
+{
+  "presets": ["stage-3"]
+}
+```
+
+### Via CLI
+
+```sh
+$ babel script.js --preset stage-3
+```
+
+### Via Node API
+
+```javascript
+require("babel-core").transform("code", {
+  presets: ["stage-3"]
+});
+```