Add LICENSE file with GPL 2, and update README

[fingolfin]

The README now points to the GitHub issue tracker, states the
license, and uses Markdown sections.

Resolves #33

[gaehler]

You may want to replace, in the license file, all ocurrencies of NormalizInterface by CARAT. Apart from that, I appreciate this PR. This is then probably a good point to wrap the thing and publish a new release of my GAP package Carat. Or should I wait for further cleanup? I do not want to miss GAP 4.11.

[fingolfin]

Oops, that was a stupid mistake. Fixed the LICENSE file.

As to making a release: yes, we should get this into GAP 4.11, on which @alex-konovalov is working right now (so we should coordinate with him). That said, of course it'd be kinda nice if the new build system got in as well (see PR #36); then again, since that is a fairly big change, and may require changes in the GAP package carat as well, maybe that's actually a reason to make a release first (to have a stable reference), and only then merge PR #36 (which is not quite ready right now anyway).

As a sort of middle ground, I could put the warning fixes and other code tweaks from PR #36 into a separate PR.

[gaehler]

I would say that I should publish a new release including all low hanging fruit soon, to be sure not to miss the train for GAP 4.11. @alex-konovalov: what is the schedule for GAP 4.11? When the rest gets ready in time, too, I can still make another release.

[gaehler]

Changes are fine with me now.

[DominikBernhardt]

Thanks @fingolfin !

[olexandr-konovalov]

@gaehler @fingolfin I'd like to finalise packages for GAP 4.11 ASAP - realistically, next couple of weeks. I really want to get 4.11 out this year, and not to do it on the last working week of December ;-)