misc: Add blurb to README.md intro

Change-Id: Ic9174d6c3d905f85e2f7f159478d8c93ef7e4079

README.md

@@ -1,8 +1,14 @@
 # The gem5 Simulator
 
-This is the gem5 simulator.
+This is the repository for the gem5 simulator. It contains the full source code
+for the simulator and all tests and regressions.
 
-The main website can be found at <http://www.gem5.org>
+The gem5 simulator is a modular platform for computer-system architecture
+research, encompassing system-level architecture as well as processor
+microarchitecture. It is primarily used to evaluate new hardware designs,
+system software changes, and compile-time and run-time system optimizations.
+
+The main website can be found at <http://www.gem5.org>.
 
 ## Getting started