Add note about need for lxd init (#11822)

* Add note about need for lxd init

As a first time user, I got a little lost, doing google searches for how to get lxd basically functional on my box. 
There were a lot of hand edits and tweaks suggested on various pages, until I discovered that all I had to do is run "lxd init" and follow the prompts.

* Update local.md

content/en/docs/getting-started-guides/ubuntu/local.md

@@ -18,6 +18,15 @@ sudo snap install conjure-up --classic
 sudo usermod -a -G lxd $(whoami)
 ```
 
+If you have never run or configured lxd before, you will need to run the following
+command to set up the default storage pool and the network bridge required:
+
+```
+sudo lxd init
+```
+
+If a bridge named `lxdbr0` already exists, you can tell the init config tool to use it.
+
 Note: If conjure-up asks you to "Setup an IPv6 subnet" with LXD, answer NO. IPv6 with Juju/LXD is currently unsupported.
 If you already have a bridge configured, e.g. `lxdbr0`, [disable IPv6 on the bridge](https://docs.conjure-up.io/stable/en/troubleshoot#common-problems), otherwise you won't be able to choose it.
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