Npm's plugin documentation (#6864)

* Documentation for Npm plugin added

* Fix style and add alias descriptions

plugins/npm/README.md

+## npm plugin
+
+The npm plugin provides completion as well as adding many useful aliases.
+
+To use it, add npm to the plugins array of your zshrc file:
+```
+plugins=(... npm)
+```
+
+## Aliases
+
+| Alias   | Command                      | Descripton                                                      |
+|:------  |:-----------------------------|:----------------------------------------------------------------|
+| `npmg`  | `npm i -g`                   | Install dependencies globally                                   |
+| `npmS`  | `npm i -S`                   | Install and save to dependencies in your package.json           |
+| `npmD`  | `npm i -D`                   | Install and save to dev-dependencies in your package.json       |
+| `npmE`  | `PATH="$(npm bin)":"$PATH"`  | Run command from node_modules folder based on current directory |
+| `npmO`  | `npm outdated`               | Check which npm modules are outdated                            |
+| `npmV`  | `npm -v`                     | Check package versions                                          |
+| `npmL`  | `npm list`                   | List installed packages                                         |
+| `npmL0` | `npm ls --depth=0`           | List top-level installed packages                               |
+| `npmst` | `npm start`                  | Run npm start                                                   |
+| `npmt`  | `npm test`                   | Run npm test                                                    |
+| `npmR`  | `npm run`                    | Run npm scripts                                                 |
+| `npmP`  | `npm publish`                | Run npm publish                                                 |
+| `npmI`  | `npm init`                   | Run npm init                                                    |