[exim-dev] GitHub membership, etc

If you're not a member of the Exim organisation on Github, but are a
committer, ping me to fix that please.

If you are a member and can't publicise the membership yourself, let me
know if you want it public. Most people are defaulted to private,
simply out of a respect for some privacy. Let me know, I'll toggle your
status to public.

I'm currently an Owner of all organisations which I'm a Member of, so
don't know whether this visibility is something regular members can
change. Oops. I just realised this might be a problem, while wondering
why so few members are set to Public.

-Phil