Add a note explaining that junction elements must be explictly tracked

With the current implementation, executing bst --track-all ELEMENT within a project which contains a junction element (an element which represents a BuildStream sub-project), will not update the ref of the junction before it starts fetching the sources of the elements within the sub-project.

I.e. You will not get the most up-to-date version of the sub project (if it has changed...).

This MR adds a note to the junction.py file which informs a user that the junction element must be explicitly tracked.

Here is the result: