README: ninja does *not* run mojob gn or equivalent.

R=jimbe@chromium.org

Review URL: https://codereview.chromium.org/1176923002.
diff --git a/README.md b/README.md
index e15c20c..33af3c6 100644
--- a/README.md
+++ b/README.md
@@ -144,8 +144,9 @@
 $ gclient sync
 ```
 
-You do not need to rerun `gn gen out/Debug` or `mojo/tools/mojob.py gn`. Ninja
-will do so automatically as needed.
+You do not need to rerun `gn gen out/Debug` - ninja does so automatically each
+time you build. You might need to rerun `mojo/tools/mojob.py gn` if the GN
+flags have changed.
 
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