Release notes documentation:

• r347037, virtual machine image reduction, and note how to resize the images.
• r346479, init_exec kenv(1) variable addition.
• r346518, spi(8) addition.
• r348482, deprecated IPSec algorithm warnings.
• r347962, driver deprecations.

Approved by: re (implicit)
Sponsored by: Rubicon Communications, LLC (Netgate)