INSTALL: Advise use of non-native SSL for Windows <= XP

Advise that WinSSL in versions <= XP will not be able to connect to
servers that no longer support the legacy handshakes and algorithms used
by those versions, and to use an alternate backend like OpenSSL instead.

Bug: https://github.com/bagder/curl/issues/253


Reported-by: zenden2k <zenden2k@gmail.com>