Add ADOPTERS.md (#1727)
Add ADOPTERS.md (#1727)

Required for CNCF graduation.

Add a list of organizations using Linkerd.

Fixes #964

Signed-off-by: Andrew Seigner <02e0a999c50b1f88df7a8f5a04e1b76b35ea6a88@sig.gy>
BuoyantIO/linkerd,linkerd/linkerd,linkerd/linkerd,BuoyantIO/linkerd,linkerd/linkerd