Update Installation Instructions

*I'm submitting a ...*
- [x] bug report
- [ ] feature request
- [ ] support request

## General information

### Prerequisites

* [x] Are you running the latest version?

### Description

The Installations instructions tell to use `wget http://nmacleod.com/public/libreelec/getwidevine.sh`: https://github.com/asciidisco/plugin.video.netflix/wiki/Installation
but this comment https://github.com/asciidisco/plugin.video.netflix/issues/438#issuecomment-414060691 claims that it is better to use the `script.module.inpustreamhelper` add-on.

### Steps to Reproduce

1. Take a look here: https://github.com/asciidisco/plugin.video.netflix/wiki/Installation
2. and here: https://github.com/asciidisco/plugin.video.netflix/issues/438#issuecomment-414060691

**Expected behavior:** [What you expected to happen]
Maybe update the installation instructions with instructions, where to find this add-on of the commenter is right.

**Actual behavior:** [What actually happened]
Don't know, what is the up-to-date way of installing widevine

## Context (Environment)

### Installation

* [ ] I installed the plugin via zip from the Releases page
* [ ] I´m using the Netflix Repo
* [ ] I´m using a different source (Please tell which)

### Operating System

* [ ] Linux (x86/x64)
* [ ] OSX (x86/x64)
* [ ] Windows (x86/x64)
* [ ] Linux (ARM)
* [ ] Android

#### Additional informatin on the environment

[Descripe your environment a bit more detailed, are you using LibreElec f.e.]

## Debug log

[Please include a link to your debug log (use something like [http://sprunge.us/](http://sprunge.us/)) or similar, please do not paste]

## Other information

[e.g. detailed explanation, related issues, suggestions how to fix, links for us to have context, etc.]

### Screenshots

[Please add a screenshot if that helps understanding your problem]

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