Add a general maintainer role for the astropy core package

[Moritz Guenther]

Hi,

originally proposed by Brigitta, we want to add a roles called "general
maintainer" for the astropy core package. Detailed discussion are in
https://github.com/astropy/astropy-project/issues/136 and we also agreed
on that at the last dev telecon.

So far, we only have maintainers by subpackage. That covers many, but
not all cases of development because we also frequently have development
that is for the package as a whole or connects a number of sub-pacakges
(e.g. creating a table with a time object from a fits file with fits
header keywords for the time system). Just like the subpackage
maintainers, the general maintainers would have github merge permission
and can merge and move forward any PR, but with the expectation to reach
agreement with the subpackage maintainers if the PR has major impact on
that package (just like subpacakge maintainers can merge PRs outside
their package if they are not too big).

APE0 specifies that there should be a two week public comment period
before a new role is added, so please leave comments and help improve
the wording of the description on

https://github.com/astropy/astropy.github.com/pull/475

Yours,

Moritz