users_guide: Update relnotes for 7.8.2

Signed-off-by: Austin Seipp <austin@well-typed.com>
Update ANNOUNCE for GHC 7.8.2

Signed-off-by: Austin Seipp <austin@well-typed.com>
Set VERSION=7.8.2, RELEASE=YES

Signed-off-by: Austin Seipp <austin@well-typed.com>