Document the 5GB limit (with instructions)

[slifty]

Right now we have a 5GB limit for uploads, which is a permanent-wide constraint. Eventually we will want to resolve this (see #145 for some related discussion, although there is not an issue explicitly capturing removing this constraint)

In the shorter term rclone provides a filter that would prevent it from attempting to upload a file of that size to begin with: --max-size

Let's update our documentation to reflect the need for --max-size 5G