add swarm rules

rules/swarms.tex

+Swarms are made of dozens, or even hundreds of small creatures, all attacking and moving together.
+They crawl over characters, inhabiting the same space as them.
+
+\begin{itemize}
+  \item
+  Swarms can pull together to bunch up on a single \gls{step}, or spread across a number of \glspl{step} equal to their \glspl{hp}.
+  \item
+  Swarms can split, then rejoin, as part of a normal movement action.
+  \item
+  The \gls{tn} to attack a swarm equals a variable minus the swarm's current \glspl{hp}.
+  (When you encounters a sea of rats, it's hard to miss all of them!)
+  \begin{itemize}
+    \item
+    A successful attack inflicts 1 Damage to the swarm, as the character crushes most of the creatures on them or next to them.
+  \end{itemize}
+  \item
+  The swarm inflicts 1 fixed Damage to everyone it occupies an area with.
+  \item
+  If the swarm has more \glspl{hp} than any armour's Cover Rating, the swarm ignores \gls{dr}.
+\end{itemize}
+