Document backingstore option and highlight that it has to be set to "none" for old lxc versions [GH-277]

CHANGELOG.md

@@ -4,6 +4,9 @@ BACKWARDS INCOMPATIBILITIES:
 
   - The `sudo_wrapper` provider configuration was removed in favor of using the
     secure wrapper generated by `vagrant lxc sudoers` [[GH-272]]
+  - Support for specifying backingstore parameters from `Vagrantfile`s for `lxc-create`
+    was added and it defaults to the `best` option. On older lxc versions that does not
+    support that value, it needs to be set to `none`.
 
 FEATURES:
 

README.md

@@ -112,8 +112,6 @@ for you. However, if the container name is important to you, you may use the
 
 ```ruby
 Vagrant.configure("2") do |config|
-  config.vm.box = "quantal64"
-
   config.vm.define "db" do |node|
     node.vm.provider :lxc do |lxc|
       lxc.container_name = :machine # Sets the container name to 'db'
@@ -123,6 +121,25 @@ Vagrant.configure("2") do |config|
 end
 ```
 
+### Backingstore options
+
+Support for setting `lxc-create`'s backingstore option (`-B`) can be specified
+from the provider block and it defaults to `best`, to change it you can:
+
+```ruby
+Vagrant.configure("2") do |config|
+  config.vm.provider :lxc do |lxc|
+    lxc.backingstore = 'none' # or 'btrfs', 'lvm', ...
+  end
+end
+```
+
+For old versions of lxc (like 0.7.5 shipped with Ubuntu 12.04 by default) that
+does not support `best` for the backingstore option, changing it to `none` is
+required and a default for all Vagrant environments can be set from your
+`~/.vagrant.d/Vagrantfile` using the same `provider` block above.
+
+
 ### Avoiding `sudo` passwords
 
 This plugin requires **a lot** of `sudo`ing since [user namespaces](https://wiki.ubuntu.com/UserNamespace)