gdb-10-branch OPEN again (was: "gdb-10 branch FROZEN")

[Joel Brobecker]

Hi everyone,

now that I have the GDB 10.2 release tarballs, the gdb-10-branch
is now open for commits again.

Note that, unless we discover a signifcant issue, we're not planning
on making another release off that branch anymore.

-- 
Joel