Add images to the README

[Thihup]

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[dtruebin]

README and description in Chrome WebStore should probably be more or less in sync, so I'll mention here that it's not too clear what the "Inject!" button. I originally thought the extension was creating the button and placing it somewhere in the Jira UI, but eventually figured out it was the extension's button on the corresponding browser panel. There is some delay between clicking the button and actually seeing the result, and users may lose patience and reload the page / leave / uninstall the extension before they see it.