Add note about PCIDB_CHROOT to README

README.md

@@ -28,6 +28,17 @@ struct contains a number of fields that may be queried for PCI information:
 the term "product ID" in `pcidb` because it more accurately reflects what the
 identifier is for: a specific product line produced by the vendor.
 
+**NOTE**: The default root mountpoint that `pcidb` uses when looking for
+information about the host system is `/`. So, for example, when looking up
+known PCI IDS DB files on Linux, `pcidb` will attempt to discover a pciids DB
+file at `/usr/share/misc/pci.ids`. If you are calling `pcidb` from a system
+that has an alternate root mountpoint, you can set the `PCIDB_CHROOT`
+environment variable to that alternate path. for example, if you are executing
+from within an application container that has bind-mounted the root host
+filesystem to the mount point `/host`, you would set `PCIDB_CHROOT` to `/host`
+so that pcidb can find files like `/usr/share/misc/pci.ids` at
+`/host/usr/share/misc/pci.ids`.
+
 ### PCI device classes
 
 Let's take a look at the PCI device class information and how to query the PCI