Implement an optional registry of host companies running PBS

[bretg]

Discussed in Prebid Server committee the desire to have a list of companies who have implemented Prebid Server:

• create a mailing list to announce bugs, features, deprecations
• enable bid adapters to contact host companies for operational reasons

Proposal:

1. We'll create a mailing list: 'pbs-host-companies@prebid.org'
2. We'll record details in a shared spreadsheet available to committee members
3. A note will be placed on the website and in the README files for PBS-Go and PBS-Java recommending, but not requiring that downloaders register for the mailing list at prebid-server@prebid.org

[bretg]

Proposed text for the README file:

Prebid does not track who downloads Prebid software, but sometimes we want to make announcements and get feedback on major changes. So we'd like to ask that you register your installation of Prebid Server by emailing prebid-server@prebid.org. Doing so will get you on a mailing list where we'll announce upcoming features, breaking changes, etc. It will be low volume and we have nothing to sell you. Here's the information we'd request:

• a group email address to place on the list (avoid individual addresses please)
• which version of Prebid Server are you running: PBS-Go or PBS-Java?
• optionally a host location so we can occasionally check version numbers
• are there any features you'd like to see in Prebid Server?
• where does the documentation most need improvement?

[bretg]

Doc created https://docs.prebid.org/prebid-server/hosting/pbs-hosting.html#optional-registration

README file PRs created. Closing this tracking issue.