filesystem: Document timestamp squashing

Came up in chat.

Signed-off-by: Colin Walters <[email protected]>

docs/src/filesystem.md

@@ -25,6 +25,12 @@ For more on container builds, see [build guidance](building/guidance.md).
 The rest of this document describes the state of the system when
 "deployed" to a physical or virtual machine, and managed by `bootc`.
 
+## Timestamps
+
+bootc uses ostree, which currently [squashes all timestamps to zero](https://ostreedev.github.io/ostree/repo/#content-objects).
+This is now viewed as an implementation bug and will be changed in the future.
+For more information, see [this tracker issue](https://github.com/containers/bootc/issues/20).
+
 ## Understanding physical vs logical root with `/sysroot`
 
 When the system is fully booted, it is into the equivalent of a `chroot`.