docs: add why and usage sections

README.md

@@ -3,3 +3,51 @@
 [![Build Status](https://travis-ci.org/grvcoelho/difflint.svg?branch=master)](https://travis-ci.org/grvcoelho/difflint)
 
 :nail_care: Run eslint only in the changed parts of the code
+
+## Why
+
+[ESLint](https://github.com/eslint/eslint) is a great tool to enforce code
+style in your code, but it has some limitations: it can only lint entire files.
+When working with legacy code, we often have to make changes to very large
+files (which would be too troublesome to fix all lint errors)and thus it would
+be good to lint only the lines changed and not the entire file.
+
+[difflint](https://github.com/grvcoelho/difflint) receives a commit range and
+uses [ESLint](https://github.com/eslint/eslint)  to lint the changed files and
+filter only the errors introduced in the commit range (and nothing more).
+
+### State of the art
+
+* [lint-staged](https://github.com/okonet/lint-staged) is a similar tool that lints only the staged changes. It's very helpful for adding a precommit hook, but it cannot be used to enforce the styleguide on a Continuous Integration service like Travis, because the changes are already commited.
+
+## Usage
+
+1. Install it:
+
+  ```sh
+  npm install difflint
+  ```
+
+2. Install `eslint` and add your eslint configuration file.
+
+3. Use it:
+
+  ```sh
+  # This will lint the last commit
+  difflint lint --commit-range HEAD^..HEAD
+  ```
+
+## Examples
+
+1. Lint the last 3 commits:
+
+  ```sh
+  difflint lint --commit-range HEAD~3..HEAD
+  ```
+
+2. Lint all commits from a build in Travis:
+
+  ```sh
+  # This environment variable will be available in any Travis build
+  difflint lint --commit-range $TRAVIS_COMMIT_RANGE
+  ```