[Instant] Add Instant.pod

Just some basic information on Instants, what I could gather from S02
and S32::Temporal.

lib/Instant.pod

@@ -0,0 +1,19 @@
+=begin pod
+
+=TITLE Instant
+
+    class Instant is Cool does Real { }
+
+An C<Instant> is a particular moment in time measured in atomic seconds, with
+fractions. It is not tied to or aware of any epoch.
+
+An C<Instant> can be used to create a C<DateTime> object set to that
+C<Instant>. The pseudo-constant C<now> returns the current time as an
+C<Instant>.
+
+Basic math is defined for C<Instant>s (as well as C<Duration>s). Adding an
+C<Instant> to a C<Duration> returns another Instant. Subtracting two C<Instant>s
+will yield a C<Duration>. Adding two C<Instant>s is explicitly disallowed. All
+other operations with Instants are undefined.
+
+=end pod