Update README.md

Author: paleite

README.md

@@ -6,6 +6,8 @@ Run ESLint on your changes only
 
 Introducing new ESLint rules (or updating 3rd party configs) in a large codebase can be tedious. This plugin allows your team to ease into new rules by only linting all new/modified code, eventually migrating to the new/updated rules.
 
+It can be used in CI environments as well.
+
 ## Installation
 
 ```sh
@@ -14,30 +16,40 @@ $ yarn add -D eslint-plugin-diff
 
 ## Usage
 
-It's recommended to use [`"plugin:diff/staged"` (see config)](#config-staged-diff-staged-changes-only).
+If you want to define which commit or commit-range to diff between (useful in CI), you can set the environment variable `ESLINT_PLUGIN_DIFF_COMMIT` (otherwise the plugin will default to `HEAD`):
 
-### Config `staged` — Diff staged changes only
+```sh
+$ ESLINT_PLUGIN_DIFF_COMMIT="a8fdc20..5a9f19c" yarn run eslint .
+# or
+$ ESLINT_PLUGIN_DIFF_COMMIT="${GITHUB_SHA}.." yarn run eslint .
+```
 
-> **Useful for pre-commit hooks, e.g. when running ESLint with lint-staged**
+See [git's official documentation on the syntax](https://git-scm.com/docs/git-diff#Documentation/git-diff.txt-emgitdiffemltoptionsgtltcommitgt--ltpathgt82308203)
+
+It's recommended to use [`"plugin:diff/diff"` (see config)](#config-diff--diff-all-changes-since-head-staged-and-unstaged).
+
+### Config `diff` — Diff all changes since HEAD (staged and unstaged)
 
 Extend your config in **`.eslintrc`**:
 
 ```json
 {
-  "extends": ["plugin:diff/staged"]
+  "extends": ["plugin:diff/diff"]
 }
 ```
 
-_Equivalent to `git diff HEAD --staged`_
+_Equivalent to `git diff HEAD`_
 
-### Config `diff` — Diff all changes since HEAD (staged and unstaged)
+### Config `staged` — Diff staged changes only
+
+> **Useful for pre-commit hooks, e.g. when running ESLint with lint-staged**
 
 Extend your config in **`.eslintrc`**:
 
 ```json
 {
-  "extends": ["plugin:diff/diff"]
+  "extends": ["plugin:diff/staged"]
 }
 ```
 
-_Equivalent to `git diff HEAD`_
+_Equivalent to `git diff HEAD --staged`_