doc: add a subsection on skipping examples.
doc: add a subsection on skipping examples.

* doc/specl.md (Skipping Examples): New subsection demonstrating
how to skip examples programatically.

Signed-off-by: Gary V. Vaughan <f9023000f29773649f3850298becb9544b5fd6a9@gnu.org>

gvvaughan/specl