Add Limitations section to README

[alanking]

We should document some limitations in the storage tiering plugin that have existed for some time but should probably be stated explicitly. Here are a couple that should be included to start:

• A data object may only have replicas in 1 tiering group (i.e. No multi-group data objects) (see #232)
• Manual management of the replicas outside of the storage tiering machinery (i.e. modifying metadata, trimming, etc.) is not recommended unless necessary because it could lead to unexpected behavior

This issue is just for adding the section and this initial list, so any additions we think of later can be handled in another issue if this issue is already closed.

[alanking]

Please open a new issue or issues for additional notes. Closing.