Collaborative editing (#1660)

.env.sample

@@ -29,6 +29,10 @@ REDIS_URL=redis://localhost:6479
 URL=http://localhost:3000
 PORT=3000
 
+# ALPHA – See [documentation](docs/SERVICES.md) on running the alpha version of
+# the collaboration server.
+COLLABORATION_URL=
+
 # To support uploading of images for avatars and document attachments an
 # s3-compatible storage must be provided. AWS S3 is recommended for redundency
 # however if you want to keep all file storage local an alternative such as