update installation instructions for snap package

[christian-rauch]

I created a snap package that includes Qt Creator and the plugin as a replacement for the old binary installer: https://snapcraft.io/qtcreator-ros. This runs as a "classic" snap with full access to the host system and should therefore behave like a regular Qt Creator installation.

This PR is adding the installation instructions to the main page.

@Levi-Armstrong I am currently hosting the instructions for building the snap in a separate repository. Do you want to create a dedicated repo inside ros-industrial (e.g. https://github.com/ros-industrial/qtcreator-ros-snap) where we can maintain the instructions and update once a new Qt Creator and ROS plugin version have been released? Additionally, I can make you a contributor on the http://snapcraft.io page so that you can release new snap packages without me.

[Levi-Armstrong]

Is there a reason not host it in this repository? I currently do this in another repo which seems to work there.

[christian-rauch]

If the snapcraft.yaml is in the repo and the repo is connected to the snapcraft.io build server, then the build server will release a new version to the edge channel every time there is a new commit in the repo on the HEAD branch (devel in our case). Right now, the snapcraft.yaml has fixed versions for the Qt Creator and the plugins, so this will re-release the very same versions multiple times to the edge channel, until the snapcraft.yaml is updated with new versions after a release.

If we put the snapcraft.yaml in a separate repo, we only have to commit once we update the versions and then the edge channel will only get new updated once we test a new version.

[Levi-Armstrong]

@christian-rauch How public do you want the repo to be? We could put it in the release org here, but if you think it needs more visibility we could put it under this org.

[christian-rauch]

The repo could be anywhere as long as a couple of ros_qtc_plugin maintainers have access to update the snapcraft.yaml after and Qt Creator or plugin update.

[Levi-Armstrong]

@marip8 Are you able to create release repository in the ros-industrial-release organization for this repo and give @christian-rauch and myself maintainership?

[marip8]

I currently don't have access to add a repo. I'll look into it, but maybe @gavanderhoorn can do this in the short term

[christian-rauch]

I am going to merge the snap installation instructions now. Let me know when there is a common repo where I can push the snap package. I will then also add you as maintainer/collaborator on snapcraft.io so you can release new versions without me.