This follows #656 by mentioning OpenSSL in the README, as well as install all
license files on installation. A hand-generated LICENSE-THIRD-PARTY is also
included.
Closes#656
This also approaches #657 by mentioning that we have GPL software in the README.
Cargo will hopefully support a more complete "all source" distribution in the
future, but at this time there is not an easy way to generate a complete source
tarball via cargo.
