New social media header image available for translation

l10n
#1

Hi everyone!

After some discussions on the marketing mailing list, we've just updated
the header image we use on these social media platforms:

* https://twitter.com/libreoffice
* https://www.facebook.com/libreoffice.org/

You can see that it's based on the new icons created by Andreas Kainz,
with a simple message underneath describing what it is, who makes it,
and with a call to get involved.

If anyone wants to make a translated version for their local social
media channels, here's a version without the text:

https://wiki.documentfoundation.org/images/b/b2/Social_media_banner_blank.png

The font I used was Vegur Bold, 32pt, hex colour: #696969

There's deliberately quite a lot of space around the icons, but that's
because different social media platforms have different dimensions for
the header images, so there's room to crop it.