README: Clarify use of BSS during SPL board_init_f()

The earlier commit....

commit a5a5d997 ("spl: Allow performing BSS init early before board_init_f()")

...introduced the ability to use BSS from SPL's board_init_f() as it may
be required in certain exceptional use cases so go ahead and update the
README to reflect this change. Note that as highlighted with the changes
the use of the associated CONFIG option is generally not recommended.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Reviewed-by: Simon Goldschmidt <simon.k.r.goldschmidt@gmail.com>