Provide a documentation for installation and usage

**🆕🐥🐶 First Timers Only**

This issue is reserved for people who never contributed to Open Source before. We know that the process of creating a pull request is the biggest barrier for new contributors. This issue is for you 💝

**👾 Description of the issue**

IcedTea-Web can be installed and used in 3 different ways:

- When installing AdoptOpenJDK 8 on Windows IcedTea-Web can be selected in the installer
- When installing OpenWebStart IcedTea-Web is used internally
- IcedTea-Web can be installed as a standalone tool.

We need a short guide in markdown or asciidoc (installation.md) that gives an overview on all 3 possibilities. The [current documentation](https://github.com/AdoptOpenJDK/IcedTea-Web/blob/master/INSTALL) is totally outdated. It contains some information about the usage as a standalone tool and when having it installed together with AdoptOpenJDK but some information are just wrong. Therefore it would be the best to start from scratch with a new documentation.

**📋 Step by Step**

To solve this issue and contribute a fix you should check the following step-by-step list. A more detailed documentation of the workflow can be found [here](https://github.com/firstcontributions/first-contributions/blob/master/README.md).

- [ ] Claim this issue: Comment below.
- [ ] Fork the repository in github by simply clicking the 'fork' button.
- [ ] Check out the forked repository
- [ ] Create a feature branch for the issue. We do not have any naming definition for branches.
- [ ] Commit your changes.
- [ ] Start a Pull Request.
- [ ] Done 👍 Ask in comments for a review :)
- [ ] If the reviewer find some missing peaces or a problem he will start a discussion with you and describe the next steps how the problem can be solved.
- [ ] You did it 🎉 We will merge the fix in the master branch.
- [ ] Thanks, thanks, thanks for being part of this project as an open source contributor ❤️

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**🤔❓ Questions**

If you have any questions just ask us directly in this issue by adding a comment. You can join our community chat at [Slack](https://adoptopenjdk.net/slack.html). Next to this you can find a general manual about open source contributions [here](https://github.com/firstcontributions/first-contributions/blob/master/README.md).