Explain the problems with 6.2
Explain the problems with 6.2

- The installation page now notes that 6.2 doesn't work
- It also disclaims support of OS X, due to that MPFR assertion failure

It'd be worth making a `--threads=0` flag or similar that ran Herbie
without invoking the places mechanism at all, to work around the
multi-threading issues on OS X and 6.2.

jackfirth/herbie,jackfirth/herbie,jackfirth/herbie,jackfirth/herbie,jackfirth/herbie,jackfirth/herbie