Add image scan docs

[StrongMonkey]

#378

[nickgerace]

Perhaps we should use <tag> instead of v1.0.3

[nickgerace]

Same here with the tag, perhaps: <image>:<tag>

[nickgerace]

nit: fleet.yaml instead of fleet.yaml

[philomory]

This documentation should probably be more explicit about what a semver range is, with an example that isn't just "*" (it's worth noting that the SemVer spec doesn't include the word "range" at all, so you're left to guess whose notion of "range" we're using here). It's probably also worth explicitly calling out how prerelease versions are handled; for example, if you write >= 1.0.0, then no pre-release SemVer tag will be considered, but if you write >= 1.0.0-0 then they will be. I had to go digging through the source code to find out that we're using the Masterminds/semver library, and then go look at their readme to see how things worked.

Also I can't tell at a glance but I think this notion of semver requires that your image tags be along the lines of 1.2.3 and not v1.2.3, which may be worth noting as well.