add checkout command to BUILDING.md

Change-Id: If817b912dd016672b7920e1c2e68244d418099d9
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/64288
Commit-Queue: Bob Beck <bbe@google.com>
Reviewed-by: Bob Beck <bbe@google.com>
diff --git a/BUILDING.md b/BUILDING.md
index adc1585..eb0f800 100644
--- a/BUILDING.md
+++ b/BUILDING.md
@@ -1,5 +1,9 @@
 # Building BoringSSL
 
+## Checking out BoringSSL
+
+    git clone "https://boringssl.googlesource.com/boringssl"
+
 ## Build Prerequisites
 
 The standalone CMake build is primarily intended for developers. If embedding