[Duration] Add Duration.pod

Somewhat brief, again using information from S02 and S32::Temporal.

lib/Duration.pod

@@ -0,0 +1,19 @@
+=begin pod
+
+=TITLE Duration
+
+    class Duration is Cool does Real { }
+
+A C<Duration> represents a length of time in atomic seconds, with
+fractions. Like an L<Instant>, it is epoch-agnostic. In numeric contexts
+(situations where a C<Duration> is used as a number), a C<Duration> will return
+a either a L<Rat> or a L<FatRat> representing the number of seconds it holds.
+
+C<Duration>s can be subtracted from or added to C<Instant>s to yield another,
+new C<Instant>. A C<Duration> will come as a result of subtracting to
+C<Instant>s. A C<Duration> can also come out of mathematical operations between
+two C<Duration>s when it makes sense (namely, the addition, subtraction, or
+modulous of two C<Duration>s). The type of object returned for other numeric
+operations is currently unspecified.
+
+=end pod