2.70 release branch created; trunk open for development

[Zack Weinberg]

I have created a Git branch for bug-fix releases to autoconf 2.70;
it's called `branch-2.70` and it already includes one patch (for bug
#110396).  Depending how quickly bug reports come in over the next
couple weeks, and how difficult they are to address, I propose to put
out a 2.70.1 bug-fix release in early to mid-January 2021.

Trunk is now open for active development again.  It's been a pleasure
working with all of you on the 2.70 release; I'm going to take a break
from serious hacking on Autoconf for at least a month, but in the new
year I hope we can find some time to talk about the future of the
project.

zw