Add LICENSE to repo and mention on rendered page

[lukpueh]

fixes #24

Screenshot of footer on rendered handbook:

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Caveat:

• The added LICENSE file is a verbatim copy from the EF page. This seems okay, because the files likely won't change, so the need to sync is unlikely. Still, if preferred, I can just add a link to the upstream file in LICENSE.
• The license mentioned on the rendered page only says "EPL-2.0" (see screenshot above). Adding a link to the upstream license or included LICENSE file, would require a custom footer template. Let me know, if that's desired.