Doc: user/quick/linux mentions 32-bit compiling

NOTRY=true

Review URL: https://codereview.chromium.org/952193002
diff --git a/site/user/quick/linux.md b/site/user/quick/linux.md
index db3f16b..7b34565 100644
--- a/site/user/quick/linux.md
+++ b/site/user/quick/linux.md
@@ -57,6 +57,11 @@
 
 If you want to use Eclipse, see Creating an Eclipse Project after you have generated the makefiles.
 
+On 32-bit Linux (when `uname -m` is *not* `x86_64`), you will have to
+explicitly specify the architecture:
+
+    GYP_DEFINES='skia_arch_width=32' ./gyp_skia
+
 Build and run tests from the command line
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