Add some text to About to act as a brief introduction to the QEMU
manual and to make the about page a bit less of an abrupt start to
it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
---
 docs/about/index.rst | 17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/docs/about/index.rst b/docs/about/index.rst
index cd44456a6bf..689a9861dc3 100644
--- a/docs/about/index.rst
+++ b/docs/about/index.rst
@@ -1,6 +1,23 @@
 About QEMU
 ==========
 
+QEMU is a generic and open source machine emulator and virtualizer.
+
+QEMU can be used in several different ways. The most common is for
+"system emulation", where it provides a virtual model of an
+entire machine (CPU, memory and emulated devices) to run a guest OS.
+In this mode the CPU may be fully emulated, or it may work with
+a hypervisor such as KVM, Xen, Hax or Hypervisor.Framework to
+allow the guest to run directly on the host CPU.
+
+The second supported way to use QEMU is "user mode emulation",
+where QEMU can launch processes compiled for one CPU on another CPU.
+In this mode the CPU is always emulated.
+
+QEMU also provides a number of standalone commandline utilities,
+such as the `qemu-img` disk image utility that allows you to create,
+convert and modify disk images.
+
 .. toctree::
    :maxdepth: 2
 
-- 
2.20.1