style_guide.md: Recommend using _prefix_variables within .gni files

Change-Id: Ia59f846e51015a7075c4c8731f41818c4cb843c3
Reviewed-on: https://gn-review.googlesource.com/2740
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

docs/style_guide.md

diff --git a/docs/style_guide.md b/docs/style_guide.md
index 466f872..295d08a 100644
--- a/docs/style_guide.md
+++ b/docs/style_guide.md
@@ -284,3 +284,13 @@
 
 `foo_use_bar` - prefixes can be used to indicate a limited scope for an argument
 (e.g. `rtc_use_h264`, `v8_use_snapshot`)
+
+#### Variables
+
+Prefix top-level local variables within `.gni` files with an underscore. This
+prefix causes variables to be unavailable to importing scripts.
+
+```
+_this_var_will_not_be_exported = 1
+but_this_one_will = 2
+```
\ No newline at end of file