Document adding a prioritized task

[githorse]

Do you want to request a feature or report a bug?

feature

What is the current behavior?

The documentation shows examples of +project and an @context at task creation, but adding a priority appears to require a separate step. It turns out that it works fine to prepend it to the task when you create it:

add "(B) THING I NEED TO DO +project @context"

But I don't see that written down anywhere. Could that be added to the README and help page?

[inkarkat]

Though it can be deducted from the "it's just a text file with some special syntax", it might be a good idea to showcase that in the recorded introduction screencast.