ltsmin: make it easier to find the README
Fixes #550 (closed), reported by Daniel Stan.
- tests/ltsmin/README: Move...
- README.ltsmin: ... here.
- Makefile.am (EXTRA_DIST): Add README.ltsmin.
- README: Mention README.ltsmin.
- spot/ltsmin/spins_interface.cc: Mention README.ltsmin in the error message.
- tests/ltsmin/check.test, tests/ltsmin/check3.test: Adjust reference to README.
- NEWS: Mention this fix.
- THANKS: Add Danial.