Add notes on using ccache

* Don't provide notes for ccache on MacOS
* Provide a link on the downloading page to the initializing
  page so that users can set the correct after
  they have ccache.

Change-Id: Ice1898c7464b25e1ddba7ee494d2b063c75d612c
diff --git a/src/source/downloading.md b/src/source/downloading.md
index 174f008..6a199c7 100644
--- a/src/source/downloading.md
+++ b/src/source/downloading.md
@@ -115,6 +115,8 @@
 
     $ git tag -v TAG_NAME
 
+If you haven't [set up ccache](initializing.html#ccache) yet,
+now would be a good time to do it.
 
 # Next: Build the code #