Bug#1117722: hdf5: explain security support status in README.Debian

[Sylvain Beucler]

Hi,

I opened a MR with a README.Debian proposal about the limited security 
support status at:
https://salsa.debian.org/debian/hdf5/-/merge_requests/1
Please review :)

Cheers!
Sylvain Beucler
Debian LTS Team