Add a note (a mental note, really, on why we won't do netlink VPP interface creations

README.md

@@ -11,3 +11,34 @@ https://gerrit.fd.io/r/c/vpp/+/31122 (netlink listener)
 
 It's intended to be re-submitted for review as a cleanup/rewrite of the existing
 Linux CP interface mirror and netlink syncer. 
+
+# FAQ
+
+***Why doesn't the plugin listen to new linux interfaces?***
+
+Consider the following two commands:
+
+```
+ip link add link e0 name foo type vlan id 10 protocol 802.1ad
+ip link add link foo name bar type vlan id 20
+```
+The two effectively create a dot1ad with an outer tag of 10 and an inner tag of
+20 (you could also read this as e0.10.20). The `foo` interface is the untagged
+VLAN 10 on e0 with ethernet type 0x8aa8, and the `bar` interface carries any
+tagged traffic on `foo`, thus is ethernet type 0x8100 within the e0's ethernet
+type 0x8aa8 outer frame.
+
+It's easy to listen to netlink messages like these, but their name will in
+no way be easy to map to a VPP subinterface concept. In VPP, all subinterfaces
+are numbered on their phy, such as TenGigabitEthernet0/0/0.1000. It is not
+clear how to map the linux host interface name `foo` and `bar` to this
+numbering scheme in a way that doesn't create collissions.
+
+A second consideration is that these QinQ interfaces can be 802.1ad or 802.1q
+tagged on the outer. So what happens if after the above, a new `foo2` interface
+is created with protocol 802.1q ? VPP only allows sub interfaces to carry one
+(1) number.
+
+Rather than applying heuristics and adding bugs, it is not possible to create
+VPP interfaces via Linux, only the other way around. Create any L3 capable
+interface or subinterface in VPP, and it'll be created in Linux as well.