Improve build-from-source instructions to be clearer and also cover building a specific release of LXD.
Improve build-from-source instructions to be clearer and also
cover building a specific release of LXD.

Signed-off-by: Daniel Robbins <14a8ed0c1e6aa8f781fd4db42e758923c2fdbbe1@funtoo.org>

stgraber/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,stgraber/lxd,atxwebs/lxd,mjeanson/lxd,lxc/lxd,Skarlso/lxd,stgraber/lxd,Skarlso/lxd,hallyn/lxd,atxwebs/lxd,lxc/lxd,atxwebs/lxd,lxc/lxd,Skarlso/lxd,stgraber/lxd,hallyn/lxd,lxc/lxd,stgraber/lxd,mjeanson/lxd,hallyn/lxd,stgraber/lxd,lxc/lxd,hallyn/lxd,Skarlso/lxd,hallyn/lxd,mjeanson/lxd,hallyn/lxd,stgraber/lxd