missing target task

Documentation should be updated regarding boot v 2.6.0 release.

I found that in my project all building tasks required adding `target` task to the chain. Otherwise final jar was built within temporary `~/.boot/cache/tmp/......` directory and was not synced with the final target.

For example:

```
(deftask build-standalone
"Build standalone version" []
(comp
(pom)
(aot)
(uber)
(jar :file (format "%s-%s-standalone.jar" (name (get-env :project))
(get-env :version)))
(target)))
```