Update readme.

README.md

@@ -1,6 +1,6 @@
 # Simulating virus evolution and epidemiology
 
-![](figures/antigen.png)
+## Introduction
 
 Antigen implements an SIR epidemiological model where hosts in a population are infected with
 viruses that have distinct antigenic phenotypes.  Hosts make contacts trasmitting viruses and also
@@ -16,8 +16,12 @@ Additionally, population structure is implemented in terms of discrete demes.  C
 deme occur through standard mass action, while contacts between demes occur at some fraction of the
 rate of within deme contact.
 
+![](figures/antigen.png)
+
 -------------------------------------------
 
+## Running
+
 I haven't made a makefile.  The program can be compiled with:
 
 	javac *.java