Add README and LICENSE for installs

This adds a readme and license text to package with installs, to satisfy the requirements of applying GPL to the code.

Our libraries' licenses also need to be included with our installs. They are copy-pasta'd one after the other in a single file, LICENSE-3RD-PARTY.TXT. I'm following the practice here.

Rationale

GPL and other open source licenses require that a copy of the text is included with every software copy. It's sufficient to include with the binary install a LICENSE file containing the GPL text and a README file that states clearly that it's GPL software and where to download the source.

The issue at play is ensuring that the binary is unequivocally related to GPL, so that it's not argued legally to be disconnected.

https://www.gnu.org/licenses/gpl-faq.en.html#LicenseCopyOnly Main rationale for the COPYING and README files

https://www.gnu.org/licenses/gpl-faq.en.html#ClickThrough says that installers don't need to force users to agree with the license; I interpreted that to mean the installer doesn't need to display it on runtime either.

https://www.gnu.org/licenses/gpl-faq.en.html#SourceAndBinaryOnDifferentSites this says it's sufficient to link to the source on an internet site, rather than include a copy of source with the install.

https://www.gnu.org/licenses/gpl-faq.en.html#DistributeWithSourceOnInternet that too.