Add some notes on live migration version and SPDK usage. (#783)

Signed-off-by: John Levon <john.levon@nutanix.com>

README.md

@@ -241,6 +241,15 @@ demonstrate the benefits of shadow ioeventfd, see
 Other usage notes
 =================
 
+Live migration
+--------------
+
+The `master` branch of `libvfio-user` implements live migration with a protocol
+based on vfio's v2 protocol. Currently, there is no support for this in any qemu
+client. For current use cases that support live migration, such as SPDK, you
+should refer to the [https://github.com/nutanix/libvfio-user/tree/migration-v1]
+(migration-v1 branch).
+
 qemu
 ----
 

docs/spdk.md

@@ -96,6 +96,11 @@ Finally, type `q` in the source QEMU monitor to exit source QEMU.
 For more information in live migration see
 https://www.linux-kvm.org/page/Migration.
 
+Note that the above live migration code in `qemu` and `SPDK` relies on the older
+live migration format, this is kept in the
+[https://github.com/nutanix/libvfio-user/tree/migration-v1](migration-v1)
+branch of `libvfio-user`.
+
 libvirt
 -------