Add maintainers.txt file with names and email addresses of project maintainers to each github repo

Hyperledger projects typically have a maintainers file with a list of project maintainers and their email addresses.  Here is an example file from fabric https://github.com/hyperledger/fabric/blob/3af753eb6afb9db890f9a1c5207fe39a2b46a14e/docs/source/MAINTAINERS.rst

These files are not necessarily at the root of each project, but reflect who maintains portions of the project.

danielhardman, we should consider how we want to handle this information for Indy.