docs: remove global-install in readme

as said in the docs, In eslint v6+, users have to install the plugins locally.
refs: https://eslint.org/docs/user-guide/migrating-to-6.0.0#-plugins-and-shareable-configs-are-no-longer-affected-by-eslints-location

README.md

@@ -123,12 +123,6 @@ The maintainers of `eslint-plugin-import` and thousands of other packages are wo
 
 ## Installation
 
-```sh
-npm install eslint-plugin-import -g
-```
-
-or if you manage ESLint as a dev dependency:
-
 ```sh
 # inside your project's working tree
 npm install eslint-plugin-import --save-dev