Add documentation about replacing tokens when upgrading to Drupal 5

Patch adds this text to the README:

UPGRADING TO DRUPAL 5

Users should take care when upgrading from the 4.7 to 5.x version of forward. The
"Forward Form Default Values" at admin/settings/forward need to be updated to use
tokens with an exclamation point (!) versus a percent sign (%). For example,
instead of "%name has forwarded a page to you from %site", the text should read
"!name has forwarded a page to you from !site". Failure to update these values
will cause forward module to sent the text "as-is" without replacing the tokens.

Maybe forward.install should check and replace instances upon upgrading?