Recognize companies that use ReactiveUI

[worldbeater]

According to https://github.com/reactiveui/ReactiveUI/issues/979 large companies including GitHub, Atlassian, Amazon, Elastic and Slack use ReactiveUI in their products. Worth placing logos of these companies to ReactiveUI website main page so new users see ReactiveUI is trusted by large IT companies. As discussed in Slack, we need to ask for permissions before doing that, to avoid copyright issues. Here is a list of companies to email and ask for permissions:

• [ ] GitHub (used in their Visual Studio extension)
• [ ] Slack (used in Slack messenger for Windows Phone)
• [ ] Amazon (used in Amazon Drive)
• [ ] Atlassian (used in Source Tree)
• [ ] Elastic (used in installers)

[glennawatson]

@shiftkey Paul Betts recommended reaching out to you about this one.

[shiftkey]

@glennawatson shoot me an email (my handle @ this website) and I'll figure out who to connect you with