[eslint-config] docs: add README (#3985)

packages/eslint-config/README.md

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+<img height="204" src="https://cloud.githubusercontent.com/assets/464822/20228152/d3f36dc2-a804-11e6-80ff-51ada2d13ea7.png">
+
+# [Blueprint](http://blueprintjs.com/) [ESLint](https://eslint.org/) Configuration
+
+Blueprint is a React UI toolkit for the web.
+
+This package contains the [ESLint](https://eslint.org/) configuration for Blueprint. It enables lint rules which enforce code style conventions and enables the `@blueprintjs/eslint-plugin-blueprint` plugin.
+
+## Installation
+
+```
+yarn add @blueprintjs/eslint-config
+```
+
+## Usage
+
+Enable this configuration in your ESLint configuration file (e.g. `.eslintrc.json`):
+
+```json
+{
+    "extends": [
+        "@blueprintjs/eslint-config"
+    ]
+}
+```