Make the instructions more welcoming those with non-government emails

There are a number of influential and interested individuals who don't have government-issued email addresses e.g. retirees, collaborators in academia, concerned citizens, etc. I'd like to propose stating that we're *not* excluding them and making joining the [GitHub for Government](https://github.com/government) community a little more welcoming to those people.

Should the requests go to government@github.com, or should they be vetted openly in a public-facing Issue under the [github.com/government](https://github.com/government) organization?

I'm sure the verbiage can be improved. Pull requests welcome.

/cc @github/government