docs: Add storage limits to arch doc
Updated the architecture document to explain that if you wish to
constrain the amount of disk space a container uses, you need to use an
existing facility such as `quota(1)`s or device mapper limits.
Fixes:
# Storage
+## Limits
+
+Kata Containers is [compatible](README.md#compatibility) with existing
+standards and runtime. From the perspective of storage, this means no
+limits are placed on the amount of storage a container
+[workload](README.md#workload) may use.
+
+Since cgroups are not able to set limits on storage allocation, if you
+wish to constrain the amount of storage a container uses, consider
+using an existing facility such as `quota(1)` limits or
+[device mapper](#devicemapper) limits.
+
## virtio SCSI
If a block-based graph driver is [configured](README.md#configuration),
