Update README to reflect recent changes.
Update README to reflect recent changes.

 - Add mail and stream to the list of blocks that can be used.
 - Remove text about defining OPENSSL_IS_BORINGSSL. This is defined by BoringSSL
   itself, so doesn't need to be re-defined by the user.
 - Add minimum version of BoringSSL required.
 - Emphasise that LibreSSL is not supported, as many people don't seem to
   realise this until it fails to build.
 - Remove text about failing to build if nginx doesn't have OpenSSL support. The
   ./configure script will now detect this automatically and fail early.

grahamedgecombe/nginx-ct