README-FIPS: document the installation of the FIPS provider

README-FIPS: document the installation of the FIPS provider

Note that configuration and installation procedure has changed:

- The FIPS provider is now disabled by default and needs to
  be enabled by configuring with `enable-fips`.
- If the FIPS provider is enabled, it gets installed automatically.
  There is no extra installation step required anymore.

This is more natural and coincides with the expectation of the
user, namely "what's configured, gets installed".

Reviewed-by: Tomas Mraz <tomas@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/13684)