docs: Update inconsistent docs for filtering files (#374)

* docs: Update inconsistent docs for filtering files

The 'Filtering files' section in the readme had not been updated when the
`gitDir` option was deprecated in 5.0.

* docs: Address feedback

- Reword {project => package} root
- Add note about absolute paths.

* docs: Update filtering part

* docs: Re-word, format

* style(readme): Single line for para

README.md

@@ -132,7 +132,9 @@ To set options and keep lint-staged extensible, advanced format can be used. Thi
 
 ## Filtering files
 
-It is possible to run linters for certain paths only by using [minimatch](https://github.com/isaacs/minimatch) patterns. The paths used for filtering via minimatch are relative to the directory that contains the `.git` directory. The paths passed to the linters are absolute to avoid confusion in case they're executed with a different working directory, as would be the case when using the `gitDir` option.
+It is possible to run linters for certain paths only by using glob patterns. [minimatch](https://github.com/isaacs/minimatch) is used to filter the staged files according to these patterns. File patterns should be specified _relative to the `package.json` location_ (i.e. where `lint-staged` is installed).
+
+**NOTE:** If you're using `lint-staged<5` globs have to be _relative to the git root_.
 
 ```js
 {
@@ -147,6 +149,17 @@ It is possible to run linters for certain paths only by using [minimatch](https:
 }
 ```
 
+When matching, `lint-staged` will do the following
+
+* Resolve the git root automatically, no configuration needed.
+* Pick the staged files which are present inside the project directory.
+* Filter them using the specified glob patterns.
+* Pass absolute paths to the linters as arguments.
+
+**NOTE:** `lint-staged` will pass _absolute_ paths to the linters to avoid any confusion in case they're executed in a different working directory (i.e. when your `.git` directory isn't the same as your `package.json` directory).
+
+Also see [How to use `lint-staged` in a multi package monorepo?](#how-to-use-lint-staged-in-a-multi-package-monorepo)
+
 ## What commands are supported?
 
 Supported are both local npm scripts (`npm run-script`), or any executables installed locally or globally via `npm` as well as any executable from your $PATH.