Update linux and mac build docs with jumbo build info

This patch updates the linux and mac build instructions
to mention jumbo builds: how to enable it, and where to
find more information.

Bug: 713137
Change-Id: I95f060660ecdb071b9102c9bba2f45259b08fd77
Reviewed-on: https://chromium-review.googlesource.com/706155
Reviewed-by: Daniel Bratell <[email protected]>
Commit-Queue: Philip Rogers <[email protected]>
Cr-Commit-Position: refs/heads/master@{#507387}
diff --git a/docs/mac_build_instructions.md b/docs/mac_build_instructions.md
index 7e9b3f2..bf811b4 100644
--- a/docs/mac_build_instructions.md
+++ b/docs/mac_build_instructions.md
@@ -132,6 +132,17 @@
 rebuilds and linking faster (at the cost of not getting symbolized backtraces
 in gdb).
 
+#### Jumbo/Unity builds
+
+Jumbo builds merge many translation units ("source files") and compile them
+together. Since a large portion of Chromium's code is in shared header files,
+this dramatically reduces the total amount of work needed. Check out the
+[Jumbo / Unity builds](jumbo.md) for more information.
+
+Enable jumbo builds by setting the GN arg `use_jumbo_build=true`.
+
+#### CCache
+
 You might also want to [install ccache](ccache_mac.md) to speed up the build.
 
 ## Build Chromium