Correct the backwards compatibility statement for v1.5.

Also mention issues with threads and 5.8.
Put the COMPATIBILITY section in a better location.

After SEE ALSO is too late.

Also integrate the compatibilty section from the NOTES section.
Change compatibility info to use stable versions of Perl.

If you care about compatibility you're writin production code.
If you're writing production code, you care about the stable versions.

Devel and stable versions don't always follow a linear relationship.
For example, 5.8.9 has 0.80 but 5.9.0 is actually older and has 0.47.

Also link to Module::CoreList which has the corelist program.