message: Update snap/README for Snapcraft 3
diff: +This is the snap package definition to build a snap package of Poedit.
  - Introduction
  -================
  -
  -This is the snap package definition to build a snap package of Poedit
  -
  -
  - Using
  ----------
  -
  -Docker doesn't work due to use of symlinks; you have to use LXD. Don't use VM
  -shared folders or remote mounts either; the only reliable way to build is by
  -doing it on native filesystem.
  -
  -1. Install LXD containers per https://docs.snapcraft.io/build-snaps/build-on-lxd-docker
  - (expect a few failures and repeating commands here, as well as having to manually get
  - apt into a sane state so that it doesn't ignore universe).
  -
  - $ lxc launch ubuntu:16.04 snapcraft -c security.privileged=true
  - $ lxc config device add snapcraft homedir disk source=$PWD path=$PWD
  -
  - $ lxc exec snapcraft -- snap install snapcraft --classic
  -
  - $ lxc exec snapcraft -- sh -c "apt update"
  -
  -
  -2. Run the build:
  -
  - $ lxc exec snapcraft -- sh -c "cd $(pwd); snapcraft"
  -
  -
  +To build it, use Snapcraft >= 3 and run the "snapcraft" command without
  +arguments in Poedit's root directory.
  