Have mailing list and community meetups visible from the webpage.

[MatthewKhouzam]

Would increase visibility of community efforts.

[planger]

Meetups and developer mailing list now mentioned on readme and web page:

• https://github.com/eclipse-cdt-cloud/cdt-cloud#-meet-the-people-behind-cdtcloud
• https://www.eclipse.org/cdt-cloud/contact/