doc: clarify semantic versioning policy (#688)

The oonimkall package is only public for technical reasons. We
cannot use `go mobile` on a private package. We consider oonimkall
our private interface to our mobile apps, thus we reserve the
right to change its API without bumping the major number.

We'll bump the major number in case of breaking changes in the
cmd/ooniprobe CLI interface, or in case of other major improvements
that significantly modify cmd/ooniprobe.

pkg/oonimkall/task.go

@@ -9,6 +9,14 @@
 // reimplementation of the mkall API implemented by Measurement Kit
 // in, e.g., https://github.com/measurement-kit/mkall-ios.
 //
+// Semantic versioning policy
+//
+// This package is public for technical reasons. We cannot use `go
+// mobile` on a private package. Yet, we are not going to bump this
+// repository's major number in case we change oonimkall's API. We
+// consider this package our private API for interfacing with our
+// mobile applications for Android and iOS.
+//
 // Task API
 //
 // The basic tenet of the task API is that you define an experiment