Added a few lines to the README.

README.md

@@ -5,3 +5,12 @@
 [![Coverage Status](https://coveralls.io/repos/ExpandingMan/Sim6502.jl/badge.svg?branch=master&service=github)](https://coveralls.io/github/ExpandingMan/Sim6502.jl?branch=master)
 
 [![codecov.io](http://codecov.io/github/ExpandingMan/Sim6502.jl/coverage.svg?branch=master)](http://codecov.io/github/ExpandingMan/Sim6502.jl?branch=master)
+
+A simulator of the venerable MOS 6502 microprocessor written in pure Julia.
+
+A couple of things to note:
+
+- This is a full-blown simulator of the 6502, not a performance-oriented emulation.  Of course the 6502 came out in 1975 so in theory most modern systems
+    shouldn't have much trouble running full-blown games with this simulator (if I ever get that far).  However, poor performance should not be interpretted as
+    poor performance of the wonderful and very performant Julia language!
+- Currently I'm making no effort to make this simulation generic.  It is designed only for the 6502.