CONTRIBUTING: Explain reasons for track dependencies on this repo (#1406)

History
=======

When discussing creating a new exercise, there's the question of any
timing dependencies between the problem-specifications repo and the
track-specific repos.

Recall that it was once required that the problem-specifications PR be
merged first, because READMEs are generated; if the problem
specification did not exist, the README generation would fail and the
student experience would be negatively impacted.

Since exercism/meta#15, READMEs are no longer
generated but included in the track-specific repos, so the absolute
dependency ("must not") no longer applies.

Discussed in #1384

Change and motivation
=====================

Change the "must not" to a suggestion.

I've written down all reasons known to me that a contributor should wait
to merge a track-specific PR.

Knowing why a suggestion exists is necessary to understand when it is
and is not okay to ignore the suggestion.

Demonstration that this is safe
===============================

I present empirical proof that there is no longer any dependency: I have
arbitrarily selected an exercise that does not have a corresponding
problem-specifications directory and does not have description.md nor
metadata.yml in its .meta directory:
https://github.com/exercism/ballerina/tree/master/exercises/hello-world-service

To the existing tooling, this is indistinguishable from an exercise for
which the track-specific PR was merged before the problem-specifications
PR.

I have personally confirmed that I can join the track, download this
exercise, read the README, implement a solution, and submit this
solution.

Author: petertseng

CONTRIBUTING.md

@@ -234,8 +234,11 @@ A problem must have a unique slug. This slug is used as
 
 * Do the same as when [porting an
   exercise](#porting-an-exercise-to-another-language-track).
-  Reference the PR in problem-specifications if it hasn't been merged yet,
-  this **must not** be merged until the exercism/problem-specifications PR is merged.
+  Reference the PR in problem-specifications.
+  It's suggested, but not required, to wait until the problem-specifications PR is merged before merging the track-specific PR, for the following reasons:
+    * If changes are suggested to the problem-specifications PR, it is likely that they will be applicable to the track-specific PR as well.
+    * Only applicable if the exercise needs a custom `title` in metadata.yml (as described in https://github.com/exercism/docs/blob/master/you-can-help/make-up-new-exercises.md#problem-specification): The title of the exercise as displayed on the website will use the default algorithm instead of the correct title until the problem-specifications PR is merged.
+    * Only applicable to tracks that have specific tools that depend on exercises being present in problem-specifications (see the track-specific documentation for whether any such tools exist): Such tools may operate unexpectedly if the exercise does not yet exist in problem-specifications. Try checking out the branch on a local copy of problem-specifications or rerunning the tool after the problem-specifications PR is merged if applicable.
 
 ## Track Anatomy