Adding an Icon to the Snap codebase

[mario-christopher]

The current version of the Snap Template does not have an Icon as part of the generated mono-repo.
Earlier, an Icon was included as part of the code base, and the builder was aware of this icon. Further, it was easier for them to replace the Icon with their product icon as it was just a replacement of an existing resource.

However, now, with removal of the Icon from the generated mono-repo, the builder no longer knows where they should put an Icon and how to do it. The bright side of this though is that we don't have issues with builders not updating the default icon, and shipping their Snap with the default Icon.

In any case, the builder needs to be aware of how to add an Icon to the Snap Project.
The foll. are some guidelines for documenting this:

• Full spec for the Icon dimensions
• The Icon should be shown properly in Light/Dark themes as chosen by the end-user
• The location of the Icon file in the project.
• The files that need to have a reference of the Icon file.
• An error emitted by the CLI if the icon file is not included (This exists today)
• The above error should contain a link leading the builder to the new Document outlining the Icon instructions.