Provide a documentation for the native build

**🆕🐥🐶 First Timers Only**

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**👾 Description of the issue**

IcedTea-Web can be build as a standalone tool. The native builds are already triggered by Jenkins:

- https://ci.adoptopenjdk.net/job/icedtea-web_build_x86-64_mac/
- https://ci.adoptopenjdk.net/job/icedtea-web_build_x86-64_linux/
- https://ci.adoptopenjdk.net/job/icedtea-web_build_x86-64_windows/
- https://ci.adoptopenjdk.net/job/icedtea-web_build_x86-64_portable/

We need a documentation of this builds in markdown or asciidoc. The [current documentation](https://github.com/AdoptOpenJDK/IcedTea-Web/blob/master/INSTALL) is totally outdated. Therefore it would be the best to start from scratch with a new documentation. Some information for the windows build can be found [here](https://github.com/AdoptOpenJDK/IcedTea-Web/pull/520). While we currently build IcedTea-Web with Jenkins the documentation must provide a way to build the native executables by hand.

**📋 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.
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- [ ] 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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