Merge pull request firecracker-microvm#129 from sipsma/master

Add note about DM_DISABLE_UDEV to README.

Author: sipsma

snapshotter/README.md

@@ -1,3 +1,12 @@
+## Requirements
+
+### Running
+Due to its dependency on `dmsetup`, executing the snapshotter process in an environment where a udev
+daemon is not accessible (such as a container) may result in unexpected behavior. In this case, try executing the
+snapshotter with the `DM_DISABLE_UDEV=1` environment variable, which tells `dmsetup` to ignore udev and manage devices
+itself. See [lvm(8)](http://man7.org/linux/man-pages/man8/lvm.8.html) and
+[dmsetup(8)](http://man7.org/linux/man-pages/man8/dmsetup.8.html) for more information.
+
 ## How to run snapshotters benchmark
 
 - `firecracker-containerd` project contains CloudFormation template to run an EC2 instance suitable for benchmarking.