Update documentation on how to configure and build the simple_build example

Change-Id: I50e6a14cfcbdf92083fb1e3f97981dc3402eb9f4
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/6560
Reviewed-by: Petr Hosek <phosek@google.com>
Reviewed-by: Wayne Piekarski <waynepie@google.com>
Commit-Queue: Petr Hosek <phosek@google.com>

README.md

diff --git a/README.md b/README.md
index a8d9fa3..fae54e6 100644
--- a/README.md
+++ b/README.md
@@ -38,6 +38,13 @@
 There is a simple example in [examples/simple_build](examples/simple_build)
 directory that is a good place to get started with the minimal configuration.
 
+To build and run the simple example with the default gcc compiler:
+
+    cd examples/simple_build
+    ../../out/gn gen -C out
+    ninja -C out
+    ./out/hello
+
 For a maximal configuration see the Chromium setup:
   * [.gn](https://cs.chromium.org/chromium/src/.gn)
   * [BUILDCONFIG.gn](https://cs.chromium.org/chromium/src/build/config/BUILDCONFIG.gn)